first amp build - choice of pre-amp and some really stupid questions

[J M Fahey]

Well, if you don't mind being pestered by the "Society Preventing Cruelty against Reverb Tanks" , you can search here and build the Phabb Reverb, spring crushing drive from a +/-35V rails    discrete power transistors dedicated power amp.

Reverb deeper than the Marianas Trench.

[incurably_optimistic]

I'm afraid a little cruelty is what this tank needs, because the op-amp drive circuit barely made it move! When I used those little transformers I have the sound was louder, but also distorted as hell.

Other thing is that the clean sound it made was not very good at all. I think it may be mostly because of small power of the driver, but I'm afraid that the tank may not be the best one.

So I think that I'll leave the reverb stuff for the future projects (maybe I'll just make it a standalone box with the mentioned Phabb Reverb drive? I'm not sure if it's worth making the +-35V power supply for the reverb only) and focus on just making the amp work.

So with the preamp done it's time for the power amp - are there any exceptional circuits involving this chip or should I just go with the ESP one: http://sound.westhost.com/project19.htm ?

It would be great if there was some well-designed, ready-to-transfer pcb out there - do you know of any? I can try to design my own, but the power amp section is too easy to mess up by a beginner I think, for example by putting the audio path to close to the rails or something stupid like that.
Or maybe I'll just spend some time googling the design rules for power amps, come up with the design, post it here and you guys will be kind enough to tell me if it's usable or a worthless crap?

[incurably_optimistic]

sorry for double posting, but I just finished a PCB design for the power amp. I know it's not pretty, but I was just trying to follow some simple rules laid out in the 3886 datasheet - keep the traces short, avoid capacitative coupling by keeping safe distance between +-35V rails and the audio path and also keep the high and low power audio lines separate. I also put the ground paths in the proximity of "high risk" signals. Oh, and all the ground paths meet at one point (BTW, should I make it smaller or shouldn't it make a problem?). I also kept the 100nF power supply caps as close to 3886 as possible.

If you see any terrible mistakes in this design please let me know!

[phatt]

I'm afraid a little cruelty is what this tank needs, because the op-amp drive circuit barely made it move! When I used those little transformers I have the sound was louder, but also distorted as hell.

Other thing is that the clean sound it made was not very good at all. I think it may be mostly because of small power of the driver, but I'm afraid that the tank may not be the best one.

So I think that I'll leave the reverb stuff for the future projects (maybe I'll just make it a standalone box with the mentioned Phabb Reverb drive? I'm not sure if it's worth making the +-35V power supply for the reverb only) and focus on just making the amp work.

So with the preamp done it's time for the power amp - are there any exceptional circuits involving this chip or should I just go with the ESP one: http://sound.westhost.com/project19.htm ?

It would be great if there was some well-designed, ready-to-transfer pcb out there - do you know of any? I can try to design my own, but the power amp section is too easy to mess up by a beginner I think, for example by putting the audio path to close to the rails or something stupid like that.
Or maybe I'll just spend some time googling the design rules for power amps, come up with the design, post it here and you guys will be kind enough to tell me if it's usable or a worthless crap?

I admire you entusiasium but maybe slow up a bit. From reading your postings you are going in guns blazing but you might be missing things. The Reveb built corectly with the right circuit will not distort. That transformer has to be that particular type and is used in this case reverse. (As Rod E explains clearly on his site).

This is the way I built it and once I realized how well it worked I mentioned it to Rod at ESP because He actually helped me create my first attempt (The one JMFahey mentioned above,, thanks for the plug Juan yes the "Maxiverb" was over the top,,  :loco)  Yes tiss rather complex,, Trans Verb is the better option as it runs from 15 V rails the transformer gives it the missing jump start to drive the tank.

Rod being far more tech savvy with the inner secrets of electronics refined it even further. He did mention that the twin opamp drive needs to be NE5532 for best performance with my design.
(or is it NE5534,, for the dual package?)

If the drive transducer in your tank measures 60 Ohms DC resistance then you should have fairly good clean sound.
Sure ideally it would be better if it was higher but I have driven a Fender Rev tank which measures 60 Ohm so I know it works ok.
Be aware you must use the whole circuit otherwise it may not work so well.
The mixer section Rod suggests on his site is an important part of the sonic result.
Although my circuit looks a bit different the *mixer part* is the same as Rod's, I've just changed the PU end and added a tone cut.


pic host

This is an inside view of the Tverb,,in a 19" rack case that came with the transformer which made the whole build much easier.


A little point I've made a few times,, It took me many years to build the Maxiverb,, many of the reverb circuits found in magazines and then ones I found on the internet I have built or at least breadboard tested.
ESP reverb circuit was the first one the worked as one would expect it to work. A lot of the others that made grand claimed turned out to be problematic. Yes it took me about 8 years to get there but I'm glad I took that journey as I've learned so much more about the many aspects of Amplification be cause I persisted and insisted that It must be doable.

But you will find it takes time to find all the hidden secrets..... Hint 8|

Phil.

[Enzo]

Look at most any Peavey amp of the last 30 years, and you will almost always see one dual op amp running the reverb pan.  it works, it is reliable, it is simple.  And they run them on single supply in their tube amps for the most part.

[phatt]

Look at most any Peavey amp of the last 30 years, and you will almost always see one dual op amp running the reverb pan.  it works, it is reliable, it is simple.  And they run them on single supply in their tube amps for the most part.

Yes indeed they do,, as does my Laney. No doubt some work better than others but the Xformer makes a huge impact on the result.
Obviously you chaps will see far more amplifiers than myself but the ones I've had experience with are not that good. My laney circuit has been modified with a small Xformer and with only slight mods needed to change it the result was a night and day difference.  I always needed to run the reverb knob up past half way to hear any decent effect,, now it's dripping wet at half way on the dial. Past 3/4 it's slapping surf twang. <3)
Phil.

[incurably_optimistic]

I'm afraid you're right phatt, I gave up fighting with the reverb tank way too quickly. But I was really annoyed, because it's third or fourth time I try to make the damn thing work and everytime I fail miserably.

I know that the transformer should be of particular type, I just tried all the transformers I have around and apparently none of them was suitable for this circuit. Right now I'm looking for the 8R/1k transformer, but can't find it anywhere in my country.

I hope it doesn't take me 8 years as everything should be much easier with your help and all those circuits, but I'm going to make this thing work, if only because it pissed me off so much that I cannot leave it unfinished  xP I'm pretty busy with other stuff at the moment, but I'll come back to it as soon as a I get the right transformer for this

[mexicanyella]

This thread is an interesting read, and an inspiring one. Also, your writing suggests that your English is excellent; no apology needed (not that one would be needed anyway if your English was worse). Just saying. Carry on!

[incurably_optimistic]

Thanks mexicanyella! I'm not done with my spamming just yet.

I've been kind of busy lately with some non-amplifier stuff, but I've done some things here and there. First of all, I've etched and populated the PCB for the power amp section:

(sorry about the photo quality, but the the camera in my old mobile phone is pretty awful.)

I wanted to get the reverb tank running, but I cannot get the 8ohm:1k transformer anywhere in my country. I don't want to give up so quickly, so I've decided to try the discrete circuit from ESP, figure 6 on this page http://sound.westhost.com/articles/reverb.htm . The bad thing is that this circuit requires a +-35V rails voltage and I still don't have a bench supply, so I didn't know how was I gonna test it.

But hey, I realized that I have a spare +-18VAC transformer from the broken amp, so I thought that maybe I should make myself a bench supply. It wasn't gonna give me +-35V required for this project, but +-25VDC should work at least a bit.

So I've decided to make a simple regulated supply using lm317/337. Here's the schematic:


I've etched the PCB and started populating it, but then I realized that I don't have enough 2200uF caps, so I stopped there:


I don't have any enclosure for this supply yet, I guess that for safety reasons using a plastic enclosure for a PS is not recommended?

[phatt]

Re the supply circuit; C7 & C8 are way to much,, try 10uF MAX. :tu:
Large Value Caps there can actually cause problems. 8|
Phil.

[incurably_optimistic]

thanks for the tip!  :tu:
I thought you cannot go too big with the power supply caps, but apparently you can. That fixes the problem - now I do have enough 2200uF caps, so I'll solder all the components today and see if it works.

[incurably_optimistic]

and by the way - should I put the fuses between the power amp section and the +-35V rails of power supply like Rod Elliott shows on his schematic? Is it necessary if I have a fuse between the mains cord and the transformer?

[phatt]

Mains fuse is a MUST!!!!  Fire risk if something major fails. You don't want to burn your house down.  8|
Some transformers have an inbuilt fuseable link but if you are not sure use a fuse. :dbtu:


Regards rail fuses,,,,,Have a look here: http://www.ssguitar.com/index.php?topic=3989.0
I've posted a small LM1875 guitar amp down that page.
For a small amp it's not so important,,
That little Casino only has 15VDC rails,,many thousands of LM1875 amps work with no fuse on the DC rails,,, so up to you. xP
Phil.

[incurably_optimistic]

That's where the problems with english not being my first language appear!

I don't want to burn my house down! (I live in a block of flats, so a lot of homes would be burned that way, and I don't want to do that! :trouble )
What I meant is that of course I am going to put a mains fuse there (in fact it's already in the power switch), but the question was: I have a mains fuse, do I need the fuses on the +-35V rails? Sorry for the confusion!

I understand your answer as: you don't have to, if your output power is small - but with 50-70W available i think I'm gonna go with the fuses, thanks!

[J M Fahey]

Use the proper Mains fuse, you need none else since there is only one large "customer" , the power amp, so if it fails primary fuse isn guaranteed to blow.

Separate fuses make sense in multi secondary transformers, where, say, a filament/heater short may not blow the main one, but overheat one small winding until it catches fire ... literally.

Besides that, I hate DC rail fuses (unless it's a single rail amp)  because they will never ever blow both at the same time, one will protect the other, and that **guarantees** DC on the speaker, a Very Bad Thing.

Don't ask me how I learnt that, many times.   :grr