DOD 650 mini-amplifier

[Mark Hammer]

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 ( 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:

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?

[Nasse]

"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

[Dan N]

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

[mikemaddux]

WOW!  211 bucks....
Ive never seen one either...
nice.

[Mark Hammer]

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.

[Toney]

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, ?

[George Giblet]

> 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.

[Mark Hammer]

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!

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.

[ahermida]

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

[Mark Hammer]

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!

[davidallancole]

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.

[Johan]

never seen that one before..cool...so now we have a DOD distortion with tone controll to try..
j

[punkin]

Hmm...interesting...looks like a simple build. I wonder how it sounds. Any chance someone might have a clip?

[ahermida]

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!

[blackcorvo]

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

[J0K3RX]

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

[blackcorvo]

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.

[J0K3RX]

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.. 

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... 

[deadastronaut]

wtf...   really?..

[Mark Hammer]

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.