Opinions wanted - Hammond Meteor as preamp

[Mark Hammer]

I'm rebuilding a little solid-state Traynor amp into something a little more substantial.  The power amp section is an LM1875, which should give me in the vicinity of 12-15W into 8R with the power supply provided ( http://sound.westhost.com/project72.htm ).  I was going to replace the preamp with Doug Hammond's Meteor.  The reviews are favourable, the controls appropriate, the tone appropriate, and I have a wad of J201's daring me to make their little lives productive.  I also can't afford a Trainwreck amp.

The Meteor "ends" with a 500k output volume pot.  Given the power amp used:

1) Will I need something in between the Meteor and the power amp to bring the level up or is the Meteor output hot enough on its own?  Is it TOO hot?

2) Will the use of a 500k volume pot between Meteor and amp jeopardize anything?  Should another pot value be used?  Is another stage called for just to buffer things?

Finally, the PS delivers about +/-18vdc (no load).  I'll use the positive side of the supply to power the Meteor.  Looking at higher supply-voltage versions of other overdrive pedals, I see that the drain resistance has to be increased to achieve a suitable drain voltage reading.  Should 100k trimpots on each of those 5 FETs cover the range needed? (the highest value resistance shown with a 9v supply is 33k).

[Doug_H]

;lkj

[Mark Hammer]

Mark, I think the 100k trimpots should work fine. This circuit should sound pretty good at 18v methinks. I can't help you with the output levels though. Maybe RG or someone can chime in there. The meteor output is pretty hot though.

What I would suggest is building your amp modularly. Do the power amp first and get it on a circuit board of its own. Then breadboard a few pedal circuits and plug them in and see what you prefer. One option may be to just build the power amp and allow yourself to plug a variety of different pedals in for different sounds.

We've had these "use a pedal circuit as a preamp" threads before.  I've always been of the opinion of allowing yourself the flexibility of plugging any pedal you want in as a preamp. Why lock yourself in to a "one trick pony" if you don't have to?

Doug

Thanks for the speedy and thoughtful reply, Doug.  I intend to assure the Meteor section is functioning fine and biased appropriately before connecting it to the power amp.

I concur with your reservations about committing to one-trick ponies.  On the other hand, when it's a 2 litre cabinet (at best) with a 6" speaker, it's not like there are a whole lot of tricks it can do.  The original was a crummy little TDA2030 10-watter with the usual 3-band EQ and gain/master controls.   What I'm keeping is the cabinet, chassis and legending, heat-sink arrangement, and PS transformer.  The guts are completely gone, with the board cannibalized for usable pieces (It had a JRC4558DD, it MUST have been good right?).

I looked through the assorted preamp and amp-emulator circuits I had (e.g., ROG), as well as a mountain of low-wattage Marshall, Fender, and Crate schems, and this seemed to possess the attributes I wanted in the most convenient package,  Soundclips sounded fine as well.

I'm perfing the whole thing, and expect there can/will be plenty of room to perf some intermediate stage should the need be apparent.  I may sink a couple of holes in the back panel for a send-return loop or perhaps a tremolo.

Because I had a bunch of nice compact 1000uf/25v caps that needed using up, I whipped up a slightly different PS.  The transformer goes to the standard 4-diode rectifier and pair of big electrolytics for a bipolar output.  Since 1000uf isn't huge, I tacked another diode (1N4002) on the outputs of the PS and ran another pair of 1000uf's to ground from there.  It drops the supply voltage down a half volt per side, and the cascaded caps functions like a 2-pole lowpass filter, rather than having one big cap acting as a 1-pole.  There is no additional regulation.  I haven't measured ripple yet, but it ought to help out a bit.  At the very least, I can sleep soundly knowing that the big bag of big caps I bought for $5 a few years ago was eventually handy.

[Torchy]

crp

[Doug_H]

;lkj

[PB Wilson]

I'm home sick with the flu today so I've been hanging out here a little in between loads of laundry.

Doug

Ahh, the glamorous life of an effects guru! :wink:

Actually I've been meaning to build up your Meteor and Highway 89 for a while now. I've got the 89 board finished and the offboard wiring started. A few more sessions at the bench and it should be ready for boxing up. Thanks for the cool pedals and get well soon.

[Doug_H]

;lkj

[Nasse]

Torchy wrote

Mark Hammer wrote:
The original was a crummy little TDA2030 10-watter  

I got a couple of these from a salvage pcb - are they poor amps ? Shame to waste em ...

What I have read TDA2030 is considered good amp chip. I have done one  (or indeed two, it is stereo but not yet in case :oops: ) amp done with two TDA2030´s boosted with medium power transistors in a bridge confiquration, and the sound is nice for such cheap and easy design. I believe the transistors I used are not matched but it works nice. Of course nowadays there are even better chips around.

Over the years I have looked for some diy guitar amp projects in electronics magazines. Some solid state projects have had some kind of filter stage between preamp and power amp. If such thing is needed, like for treble and bass rolloff and some general basic tone shaping, if you go for modular design, it can easily added if necessary.

[David]

Torchy wrote

Mark Hammer wrote:
The original was a crummy little TDA2030 10-watter  

I got a couple of these from a salvage pcb - are they poor amps ? Shame to waste em ...

What I have read TDA2030 is considered good amp chip. I have done one  (or indeed two, it is stereo but not yet in case :oops: ) amp done with two TDA2030´s boosted with medium power transistors in a bridge confiquration, and the sound is nice for such cheap and easy design. I believe the transistors I used are not matched but it works nice. Of course nowadays there are even better chips around.

Over the years I have looked for some diy guitar amp projects in electronics magazines. Some solid state projects have had some kind of filter stage between preamp and power amp. If such thing is needed, like for treble and bass rolloff and some general basic tone shaping, if you go for modular design, it can easily added if necessary.

Nasse:

Do you have a schematic for this?  I picked csj's brain about TDA2030's early in the summer, right before my wife's car accident.  He said that in his experiments, he found it very difficult to keep from overdriving the inputs of the 2030.  Obviously with everything that happened afterward, I haven't had much time to revisit this.  I was planning to do something similar to what Mark described with a 2030 if I ever get around to it.  I have the schematic for a Mar*^a@% that uses a 2030.  I figured I could always clone that if I had to.  I just thought it would be cool to get the rest of the story from someone who had done it successfully.

[b_rogers]

FWIW i have a little gem with the hwy 89 in front and it sounds great. all i did was add a 100k trim in between the 89 and the gem.

[csj]

I just thought it would be cool to get the rest of the story from someone who had done it successfully.

Yeah, me too David! (hope your wife is doing better btw)

All I can say is that I've been completely underwhelmed by both the 2003 and the 2030. I'm a little familiar with Doug's Meteor (great pedal) and I would think it would just bash the crap out of either of these setups without something to knock the signal down. I haven't used the transistor Mark is using but these other 2 seem to be of no use to me at all. Since I don't know how to accurately measure the gain of a fet (let alone a stack of them) it would mean that I'd have to again juggle these amp's circuits loop gain resistors and when I did this before I had to bring it down to such a low level to even accept a simple single coil voltage that the overall power out level would have had a hard time being heard in a hamster farting contest. When I tried to get the gain lower in the 2030 things just got weird and I shelved the whole project. I used a filtered and regulated variable supply voltage at several different test levels. Load dumped into very low efficiency 4 and 8 speakers. It was another one of my many failures.

Since that time one of the local pedal steel/tele playing hotshots here brought me a very cool amp built ages ago by Shobud. It was a single channel with reverb and had a few simple mechanical problems with it. I kept it for about a month just to play it. I liked it so much I wanted to keep it but he wouldn't sell it to me (and this is coming from a hack player who has a shamefully embarrassing number of amps too) So, well... you probably know what I did. If anyone is interested in it let me know. The problem is that the power transistors, as far as I can tell, are obsolete. It's going to need to be brought up to date that way and I haven't had the time.

Mark, not meant to hijack your thread. I'm hoping it works for you so you can show me how to do it.

[Mark Hammer]

No one's hijacked anything, so not to worry.

Re: TDA2030

I thought I had written a reply yesterday but I guess I must have closed down my browser by accident before I sent it.  When I say "crummy TDA2030-based amp", I don't mean to imply that the chip itself is lousy.  Rather, the chip is inexpensive and is generally the first choice of manufacturers who want to make VERY inexpensive first amps for the bedroom headbanger crowd.  The overall package tends not to be terribly good, with small cabs, poorly chosen speakers, insufficient current from the transformer, and what-the-hell-it's-only-two-diodes "overdrive" thrown in for good measure.  The Traynor one I cannibalized basically had a 4558, a TDA2030 and nothing else, and plenty of others from other name companies (e.g., Marshall) are similar.  Crate's and Fender's are a slight cut above, but again, the intent of these products is to make something that works okay at low production cost, not something to knock your socks off.

Chances are pretty good that a TDA2030 or similar chip COULD be made to sound decent, if married with a thoughtful preamp, decent speaker/cab, and suitable power supply.  This IS a solid state amp and the purpose is to simply make the preamp signal bigger, not to impart any distinctive tone on its own.  As long as the other parts are willing to cooperate, there is no reason why the power-amp chip should not do so either.

CSJ,

Check out the schems for the various Fender Frontman amps and Marshall Valvestates (e.g., http://www.drtube.com/schematics/marshall/8010.pdf )

[goosonique]

hey Doug...just finished the Meteor....awesome! Then ....being me ... play around a bit got few changes to suit my taste.
Just one addition tho ... 0.1u just before R9 (2.7K) to ground gives a bit more Gain i guess....using 2N5457. I flaged the R22 (1K) ...it just didn't work with 2N5457's. I guess using J201 will be better.

If this is not right pls correct me....btw any more upgrades on the Meteor?
More Gain = distortion

Thx again...