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DIY Stompboxes => Building your own stompbox => Topic started by: Mark Hammer on March 31, 2009, 07:34:57 PM

Title: DOD 650 mini-amplifier
Post by: Mark Hammer on March 31, 2009, 07:34:57 PM
I tried finding out something about this today, in response to a thread and request for help over on MEF.  When not even the discofreq site had anything about it ( :icon_eek: could that even be possible?! ), I dropped the DOD service desk a note and within an hour or so, they sent me the schematic.....and here it is:
(http://i414.photobucket.com/albums/pp228/Mark_Hammer/Dod650.png)
As you can see, it is essentially a 250 with a small 4-transistor power-amp section, powered by six AA cells.  So, basically their "Pignose" (though I've never actually seen one).  I have no idea what sort of power it might put out, but with 6AA power, it obviously won't be more than a watt or so.  The drawing shows a 16ohm load which appeals to me and the bunch of 32-ohm Macintosh speakers I have hanging around.

Having said that, there are some things about it that make no sense to me.  Chiefly, do YOU see a feedback path from the output of the 2nd op-amp?  Neither do I.  So how is gain set?  Of course, there are no power connections to the IC shown either.  And what's the deal with the path from the output back through the 470k to the 22k/.1uf pair?
Title: Re: DOD 650 mini-amplifier
Post by: Nasse on March 31, 2009, 10:02:30 PM
"And what's the deal with the path from the output back through the 470k to the 22k/.1uf pair?"

I thought this was negative feedback, but I am really bad with transistor circuits.


Lately I have been using cheap modelling preamp (with aux input for my cheap mp3 player) and when playing trough my small practice amp with 8" guitar speaker backing tracks dont sound so good so I think I want more hifi amp. Last weekend I tried trough small stereo chipamp and old boxed cheapo car stereo speakers (8" woofer with foam surrounding and piezo tweeter) and it was nice at low volume but something clipped in the chain and piezos started to sound nasty (I cranked the volume when wife was out) Perhaps I can try 3" full range bass reflex boxes I have unused in the upstairs later this week, with the cabinet modelling on.

Someone somewhere said that such cheap speakers with foam surrounding and often sold as universal replacement speakers do sound good in practice combo and I think they do
Title: Re: DOD 650 mini-amplifier
Post by: Dan N on April 01, 2009, 04:44:56 AM
Thanks for the schematic!

Funny, just last week I was looking at an ebay seller's "other items for sale", and he had this weird DOD I'd never seen before.

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=380111953991


Title: Re: DOD 650 mini-amplifier
Post by: mikemaddux on April 01, 2009, 04:53:58 AM
WOW!  211 bucks....
Ive never seen one either...
nice.
Title: Re: DOD 650 mini-amplifier
Post by: Mark Hammer on April 01, 2009, 08:59:00 AM
Thanks for the link to the E-bay ad.  very useful, and at the same time confusing.

Now here's the weird part.  Check out these two pictures:  http://music-electronics-forum.com/album.php?albumid=33&pictureid=185
http://music-electronics-forum.com/album.php?albumid=33&pictureid=186

They plainly indicate that this is a DOD "650" board, yet the component complement is onl;y tenuously related to the schematic I posted, and obviously the board is entirely different.  Weird.
Title: Re: DOD 650 mini-amplifier
Post by: Toney on April 01, 2009, 09:22:23 AM

 I'm drawing this up at the mo, Mark... been playing with Rubies and other 386 amps this week so it's piqued my interest.
 What do you suppose for the power to the TL022, ?
Title: Re: DOD 650 mini-amplifier
Post by: George Giblet on April 01, 2009, 09:30:36 AM
> And what's the deal with the path from the output back through the 470k to the 22k/.1uf pair?

Actually, it's all very normal.

The 470k *is* the feedback resistor and the 22k + 100nF cap is the usual RC to ground.  The gain is 1+470k/22k; the non-inverting gain formula. To see that the trick is to ignore the 8n2 cap + 10k + 100k tone pot - those parts act to boost the treble pretty much the same way a Presence control works on a tube amp.   In the full boost position the 10k+8n2 appears in parallel with the 22k + 100nF.   I'm sure you will see it now?  In the cut position the 22nF cap forms a low pass filter with the 10k.   The input filter and the feedback filter work in opposition to each other as far as the pot position goes.  Because the tone pot is grounded the input and feedback filtering don't interact.

Title: Re: DOD 650 mini-amplifier
Post by: Mark Hammer on April 01, 2009, 10:45:21 AM
George,

Can't thank you enough for that explanation.  Makes perfect sense now, and also gives some hints for how to tailor the tone control response to suit the speakers used (well worth doing so armed with Jack Orman's excellent documents on playing with the TS-9 tone control, which this is a variant of).  And of course, if one thinks of the 470k resistor as the feedback path, then the preamp output also makes perfect sense.

The lights come on! :icon_biggrin: :icon_biggrin:

Toney,

The TL072 can probably be any of a variety of other dual op-amps.  The 022 just consumes less current.  I can't see anything else that is critical about use of that chip.  Power goes to pin 8, of course, and pin 4 is ground.

What I'm curious about is the nature of the preamp out signal.  Much of the circuit leading up to, and including, the second op-amp has a strong resembance to the 250 and the core of the Tube Screamer, but there's all this other "stuff" between op-amp 2 and the point where the preamp output is tapped.  I'm not naive enough to believe it will be like a power-amp output, but I'm curious as to how it might be different. ???
Title: Re: DOD 650 mini-amplifier
Post by: ahermida on April 01, 2009, 05:09:11 PM
Please also look at the MXR headphone amp schematic.  You'll notice some similarities and maybe a simpler design.

http://www.diystompboxes.com/pedals/MXRHeadphoneAmplifier.bmp

The drawing incorrectly mentions an IC but if I recall correctly its the LF353.  The text in the drawing needs to be updated.

Alf
Title: Re: DOD 650 mini-amplifier
Post by: Mark Hammer on April 01, 2009, 05:53:57 PM
Hi Alfonso!

Long time no hear.

The MXR is clearly a simpler, but similar device.  I gather the extra transistor pair in the DOD ups the current to be able to power speakers, in comparison to the headphone assumptions of the MXR.  But following George's note, I recognize the same sort of feedback path for the op-amp.

Thanks!
Title: Re: DOD 650 mini-amplifier
Post by: davidallancole on April 02, 2009, 12:09:04 AM
Mini amps like this are fun to play with.  They can play plenty loud enough for practicing at home with them.  The MXR headphone one would be easy to turn into a mini amp that can play into a speaker.  Add two more diodes in the string there and make the outputs darlington connection and your good to go.
Title: Re: DOD 650 mini-amplifier
Post by: Johan on April 02, 2009, 03:16:19 PM
never seen that one before..cool...so now we have a DOD distortion with tone controll to try..
j
Title: Re: DOD 650 mini-amplifier
Post by: punkin on April 02, 2009, 03:33:06 PM
Hmm...interesting...looks like a simple build. I wonder how it sounds. Any chance someone might have a clip?
Title: Re: DOD 650 mini-amplifier
Post by: ahermida on April 02, 2009, 03:40:42 PM
Hey Mark!  I've been under a pile o' work.  Trying to get back and contribute when I can.

I actually have the original MXR headphone amp.  Many moons ago I used it as a booster.  It was noisy but it worked. 

Alf

Hi Alfonso!

Long time no hear.

The MXR is clearly a simpler, but similar device.  I gather the extra transistor pair in the DOD ups the current to be able to power speakers, in comparison to the headphone assumptions of the MXR.  But following George's note, I recognize the same sort of feedback path for the op-amp.

Thanks!
Title: Re: DOD 650 mini-amplifier
Post by: blackcorvo on July 02, 2016, 11:02:26 AM
Hmm...interesting...looks like a simple build. I wonder how it sounds. Any chance someone might have a clip?

Just to bring closure to this question, I found this video today: https://www.youtube.com/watch?v=cTd_PTkcDv8
Title: Re: DOD 650 mini-amplifier
Post by: J0K3RX on July 02, 2016, 07:02:48 PM
Hmm...interesting...looks like a simple build. I wonder how it sounds. Any chance someone might have a clip?

Just to bring closure to this question, I found this video today: https://www.youtube.com/watch?v=cTd_PTkcDv8


Probably would sound better if it had an external spkr jack and you hooked it up to a 2 x 12 cab or something...
http://www.effectsdatabase.com/model/dod/first/650
Title: Re: DOD 650 mini-amplifier
Post by: blackcorvo on July 03, 2016, 12:02:25 AM
Just to bring closure to this question, I found this video today: https://www.youtube.com/watch?v=cTd_PTkcDv8


Probably would sound better if it had an external spkr jack and you hooked it up to a 2 x 12 cab or something...
http://www.effectsdatabase.com/model/dod/first/650

I don't have one of those, but I agree. I hate how they never add a speaker out to small guitar amps! I have 2 Honeytones that I did that mod to, and there's a Benson (aka Marshall MS-4 clone) that I ordered that will most likely recieve the same treatment once it arrives to me.
Title: Re: DOD 650 mini-amplifier
Post by: J0K3RX on July 03, 2016, 06:40:19 PM
Yeah, I have a couple little practice amps that range from 1 to 15watts and they all sound amazing running into either my Marshall 4 x 12 or my Legacy 2 x 12.. both are loaded with Celestion vintage 30's..  Every time I happen to get my hands on a crappy little practice amp I bring it home and give it the 4x12 test..  :icon_twisted:

I think this video of Lasse Lammert running a Marshall MS-2 battery powered 1 watt microamp into a Mesa Recto 4 x 12 cab opened my mind to the possibilities...  :icon_twisted: :o :icon_twisted:
Title: Re: DOD 650 mini-amplifier
Post by: deadastronaut on July 04, 2016, 11:51:49 AM
wtf... :icon_eek:  really?..
Title: Re: DOD 650 mini-amplifier
Post by: Mark Hammer on July 04, 2016, 01:11:31 PM
Certainly the tonal properties of a 12" speaker will be more in keeping with what we think a guitar is supposed to sound like than a 3" speaker does/will.  But, like yourself, I am skeptical of the extent to which a measly 1W (on a good day, with the wind at your back, and fresh batteries) can move the cones of a multi-driver cab that anticipates being on the receiving end of upwards of 100W.  Keep in mind that the 1W rating is not the rating of the mini-amp's guts behaving themselves.  A well-behaved mini-amp using an 8-pin power amp chip probably won't give you more than 600mw of good behaviour, and is really not intended to, either.  Will that move the cones across the full spectrum?  Not likely.  It will sound better than a 3" speaker, but my guess is you need to listen to it, or mic, in some very special ways.
Title: Re: DOD 650 mini-amplifier
Post by: PRR on July 04, 2016, 02:33:58 PM
> need to listen to it, or mic, in some very special ways.

A 4*12 has 64 times the cone-area of one 3". Air is mushy. 64 times the area is much better grip on the air. It is not unlikely that, in mid-bass, the 4*12 with 0.6 Watts will be as loud as the 3" with 38 Watts (if it survived).

No, a flea-Watt amp into a Full Stack will not fill a stadium! But we did fill 3,000-seat stadiums (no PA) with 100-200W in full stacks. It seems reasonable that a flea into a stack could be Real Frikkin Loud in a 15-seat venue (large living room or teeny club). Without tricks.
Title: Re: DOD 650 mini-amplifier
Post by: Mark Hammer on July 04, 2016, 07:38:58 PM
Normally, I would humbly defer to you, because of your generally superior electronic knowledge andgreater mathematical dexterity.  But here I will draw attention to the relative mass of the speaker cones, and plain old vanilla inertia

My wife got us a new kitchen sink last year with "hands-free" operation.  Trouble is, the thing runs off batteries and those damn solenoids eat a LOT of current to operate.  So we just disconnected the automatic stuff and use it manually because we got tired of crawling under the counter to change the batteries.

Speakers are essentially solenoids, and it takes juice to move those bloody cones.  The bigger the voice coil and cone mass, the more current.  And if they are intended to survive the sort of current accompanying 100 or even 50W, they are not going to be particularly light or compliant.  The puny 600mw of a 386-based amplifier will certainly get FAR more bass from those speakers than from a 3" speaker in a cab 1/500th the internal volume (although in many instances the mini-amp will have a insufficiently large electro cap on the output to permit much below 200hz).  BUt that's going to be more a result of cab/speaker resonance, and the inability of the mini-amp to make the speaker move fast enough to reproduce treble.

Of course, when all is said and done, what the battery-operated mini-amp can or can't make the speaker do is completely separate from how the combo sounds.  Indeed, it may well be the case that what some folks find appealing about driving a cab meant for much greater wattage is a direct result of what the mini-amp can't do.  Heck, that principle is a big part of what rock music is supposed to sound like.  Overdrive and distortion is a product of a signal path that simply can't amplify with any degree of fidelity.
Title: Re: DOD 650 mini-amplifier
Post by: PRR on July 04, 2016, 09:52:14 PM
> those damn solenoids eat a LOT of current

Bad design?

1) I wonder if they take power to stay "open". Simple but wrong. A shuttle valve only needs a short blip to open or close, needs no power to hold its state. Like normal relays versus latching relays.

2) For a fraction of what they charged you they cudda put a mini-turbine in the water path to spin a 59-cent Mabushi motor as a generator and recharge batteries.

They are not optimizing the motor to the load and the cost of power. Such concerns are a *prime* obsession with the speaker engineer.

The ideal speaker is like many other engineering devices: 90+% efficiency. Motors and transformers and gearboxes do this routinely. Combustion engines are awkward because they don't beat 40% efficiency.

The "ideal" speaker for highish efficiency down to 50Hz must have a 120 inch (10 foot, 3m) air-smacker. If any smaller, it can't couple to air well over a useful wife freq band.

Near-Ideal speakers are impractical. (Theater speakers of the 1930s came close; so do some of the very large arrays now used in cow-palaces.)

The 3-inch cone is efficient down to 2,000Hz. Below that efficiency % drops as square of frequency. At 200Hz it is necessarily 1% efficient at best. For "flat" response (and a mass-related reason) the parameters are adjusted to be 1% efficient 200Hz-2KHz. (Above that, efficiency falls but directivity rises; on-axis response may be flat another few octaves up.)

Take a four-12 array as a 24" cone. 8 times bigger than the 3-inch. All frequency benchmarks drop 8X. It loses efficiency below 250Hz (not 2KHz). For other reasons it will never come close to 100%, even 50%, efficient. For guitar we would likely aim for 80Hz, 3X lower freq than the bass/efficiency corner, 10X lower efficiency, around 10% from 80Hz to 250Hz. This size flat array also leads to high directivity in the >250Hz range, so stunning "throw", impressive sound in-front which cuts-through reverberant spaces. Above 1KHz-2KHz the directivity pattern gets very narrow while also growing side-lobes different at every frequency, though a four-12 will never keep all four cones-phase-matched above 500Hz which mixes-up the otherwise beamy directivity.

Unless heavily slugged for deep bass, or assembled by faucet designers, the larger air-slapper will generally put out more sound per electric Watt.

You seem to be leaning on an argument that the amp power and speaker rating must be alike. The ideal speaker is perfectly linear at any power. Yes, MUCH guitar tone is from driving non-ideal non-linear speakers somewhat past what the original (non-guitar) designer ever expected. But some speakers got past the Magnavox designs. The older Altec theater woofers, E-V's EVM series, and the wonderous JBL D-120/130. Some players dislike them as "too clean". Others appreciate clean reproduction of whatever mangled tone you feed them. Running 0.6 Watts into four Brand E speakers rated 25W/each with heavy breakup is a lot like a 50W into an EVM good for 600+W clean peaks.
Title: Re: DOD 650 mini-amplifier
Post by: Mark Hammer on July 05, 2016, 09:25:54 AM
You seem to be leaning on an argument that the amp power and speaker rating must be alike. The ideal speaker is perfectly linear at any power. Yes, MUCH guitar tone is from driving non-ideal non-linear speakers somewhat past what the original (non-guitar) designer ever expected. But some speakers got past the Magnavox designs. The older Altec theater woofers, E-V's EVM series, and the wonderous JBL D-120/130. Some players dislike them as "too clean". Others appreciate clean reproduction of whatever mangled tone you feed them. Running 0.6 Watts into four Brand E speakers rated 25W/each with heavy breakup is a lot like a 50W into an EVM good for 600+W clean peaks.
I guess it may seem like that, but that's not my intent at all.  The gist of my argument is that few speakers are "ideal".  They will have a linear and a non-linear range, plus some transitional zone.  Some minimum given amount of amplifier output current/voltage is required to nudge them into their linear range, and the bigger and heavier the cone-and-coil assembly, and the stiffer the spider (in anticipation of handling BIG wattages), the more juice will be needed to move from non-linear to linear.  It doesn't have to be a LOT - which is why single-ended three-tube 5W amps driving a 4x12 cab can sound pretty darn fine - but nominal 1W amps running off a 9V battery are highly unlikely to meet those minimum requirements.

Again, what transpires when a teeny battery amp tries to push a big cab may well sound great; especially when mic'd up just right.  But the tone achieved is unlikely to be the result of any part of that signal chain operating as nature and design intended.  In other words, it's not the result of big speakers adding "more" to the mini-amp's output than the internal small speaker, the way that replacing a 10 with a 12 in a medium-powered (under 25W) amp would.
Title: Re: DOD 650 mini-amplifier
Post by: PRR on July 05, 2016, 03:39:27 PM
> few speakers are "ideal".

None.

> They will have a linear and a non-linear range, plus some transitional zone.  Some minimum given amount of amplifier output current/voltage is required to nudge them into their linear range

Ah.

The "center" of a voice coil's motion should be very darn linear. They should reproduce the smallest quiver of wide dynamic range material. No stiction.

If the linear range did not extend *through* zero, that would amount to "crossover distortion" as in transistor amplifiers. Nasty at low level.

My experience with speakers says they make no non-linear crap at levels at least 70dB down from "normal listening level". If they did, much "fine" music would sound crappy in the soft passages. I suspect any near-zero stiction happens at levels well below room ambient background noise.
Title: Re: DOD 650 mini-amplifier
Post by: Frances Rhodes on March 17, 2018, 12:56:26 PM
hi everyone

bumping this old thread.

i recently build a clone of the DOD 650 using that schematic, and it works, bad, but it works.
i build it using salvaged speakers from an old TV set and they are rated 10W each so i was looking for a way to increase power without having te redesign a new power amp section, and i looked at the schematic again and something bothers me about the 2 pairs of output transistors.
isn't the lower darlington pair strange, with the emitter of the first npn pointing "up"? or is it one of the different ways to hook 2 transistors up for more gain?
the only way of making a darlington pair i know of (but i know few...) is to tie the second base to the first emitter and tie both collectors together, so this setup doesn't look right to me. since my amp is making sound, means it works, but doesn't mean it works the way it's supposed to, so i may be wrong but i feel like this odd-looking transistor arrangement may be a reason why the sound bad.

cheers
Title: Re: DOD 650 mini-amplifier
Post by: PRR on March 17, 2018, 05:20:16 PM
Not Darlington. Sziklai (https://en.wikipedia.org/wiki/Sziklai_pair).

(https://s18.postimg.org/ejlni3idl/Dod650.png)

> it works, but doesn't mean it works the way it's supposed to

No clue. I can't see your voltages from here.
Title: Re: DOD 650 mini-amplifier
Post by: Frances Rhodes on March 17, 2018, 07:17:44 PM
i'm not talking about the sziklai pair (PNP + NPN) but the other one, the lower one (NPN + NPN) that looks like a darlington but with the first transistor upside down, with the second transistor tied to its collector instead of its emitter.

i can post voltages, but my question is about that schematic, for now.
Title: Re: DOD 650 mini-amplifier
Post by: PRR on March 17, 2018, 09:59:48 PM
I see. That's just wrong. Where did this plan come from?

Should be this.
(https://s18.postimg.org/5sntfn67p/DOD650-mod.gif) (https://postimg.org/image/5sntfn67p/)
Title: Re: DOD 650 mini-amplifier
Post by: PRR on March 17, 2018, 10:17:49 PM
me> Where did this plan come from?

http://music-electronics-forum.com/t12696/#post100112
Mark Hammer: "....here is the schematic. FWIW, I'm not so sure it is entirely accurate, even though it came directly from DOD.:"

Title: Re: DOD 650 mini-amplifier
Post by: PRR on March 17, 2018, 10:27:41 PM
> ...a clone of the DOD 650 .... salvaged speakers from an old TV set and they are rated 10W each so i was looking for a way to increase power....

9V supply into the 8+8= 16 Ohm load is only 1/2 Watt. Probably 1/4 Watt due to opamp and output losses.

The indicated 2N4124/5 transistors are very low-current jobs, already strained at this power level.

Your "10 Watt" speakers won't stand 10 Watts of distorted guitar for very long.

I do wonder if your TV speakers are 8 or 4 Ohm.

Short path to "more power": get a Car-Audio chip. Run on 9V it will make about 9 Watts in 4 Ohms (two 8r parallel), which they will probably stand. The 9V supply needs about 800mA, so a fairly hefty supply.

The preamp from the 650 will be useful.
Title: Re: DOD 650 mini-amplifier
Post by: Frances Rhodes on March 18, 2018, 03:10:14 PM
ok so you think that the problem here is that the NPN should be a PNP instead, that makes sense.

my salvaged speakers are 10W / 8 ohms each, in series to match the 16 ohms output specified.

i was thinking of keeping the same design (considering the modification you made) replacing the four output transistors by two TIP31 and two TIP32 which can handle more power, and also use a "bigger" supply, maybe 12V-1A. can this work or is it a completely wrong way of doing it?
Title: Re: DOD 650 mini-amplifier
Post by: PRR on March 18, 2018, 09:57:58 PM
12V DC allows 4Vrms swing in a load. That is 2 Watts in a 8 Ohm load or 4W in two 8r loads parallel. Naked TIP devices will just barely support the dissipation. Any short will burn the TIPs.
Title: Re: DOD 650 mini-amplifier
Post by: Frances Rhodes on March 19, 2018, 05:36:59 AM
"naked" you mean without a heat sink?
Title: Re: DOD 650 mini-amplifier
Post by: bluebunny on March 19, 2018, 10:10:20 AM
Yep.
Title: Re: DOD 650 mini-amplifier
Post by: Frances Rhodes on March 19, 2018, 12:45:44 PM
ok, right, but other than that, with a proper supply, this should work fine with just swapping the actual transistors for TIP ones?