SS Gain recovery?

[Ripthorn]

Hi guys, working on the last touches of my build. I have it set up to true bypass the tonestack, but when I engage the tonestack, the volume loss is rather dramatic. I knew there would be some volume loss, but not this much. So I wanted to put in a little SS gain recovery between the tonestack and the volume control. I used a TL071 opamp set up as an amplifier as shown on the bottom right here http://www.muzique.com/lab/boost.htm. The problem is that while it boosts the level, it also clips rather hard, which I don't want. I just want a simple, clean level increase. I am using a 12V supply (I think it is rated for a max 18V).

I was wondering if I am overloading the input, so I used the old meter to see what kind of max voltages I am getting and I get a max of about .5V peak to peak, which shouldn't be near enough to clip the opamps input. I guess I could be clipping the inverting input, but the volume increase from unity on up is gradual enough that I wouldn't have thought it.

My question then is what might be the issue. Also, if anyone has a better suggestion as to a solid state gain recovery scheme, I am willing to give it a shot. Thanks.

[Structo]

What kind of amp are you building?

Are you using a relay to switch the tone stack in and out?

The Dumble ODS has a tone stack defeat switch called PAB or preamp boost and it simply takes the tone stack out of circuit for the boost.

But what can happen depending on where you have your pre gain and master volume set is a huge boost when it is switched on.

The simple way to address it is to pad it with some resistors.

Here is the relay board for it.
It shows two 22M resistors for pads but I just installed two 10 meg resistors which I like better.
I plan on experimenting with other values.

[Ripthorn]

Thanks for the layout Tom. The amp that I am building is a submini amp using 5902's in pp for the power amp. The pad sounds like it would be fine, except that even when the amp is maxed and the tonestack is engaged, it is still quiet. I would rather bring the post tonestack signal up if possible. If I can't get that working, then I will leave it as is.

[Structo]

Well I am far from that knowledgeable on design but I do know that plate loaded tone stacks behave differently from cathode follower tone stacks so perhaps you can re-design where the tone stack is in the amp to avoid the loading down effect of the tone stack.

I am just guessing here but there has to be a way to solve your problem.

[jjman]

If the tone stack does NOT include a Mid control, you could increase the value of the "Mid resistor." This would increase all the frequencies somewhat and the mids significantly, while still allowing for tone shaping.

This assumes you are using a Fender BF/SF type stack.

If you do have a mid control, you already have the option of turning it up to reduce the volume change during switching.

[drz400]

Hi guys, working on the last touches of my build. I have it set up to true bypass the tonestack, but when I engage the tonestack, the volume loss is rather dramatic. I knew there would be some volume loss, but not this much. So I wanted to put in a little SS gain recovery between the tonestack and the volume control. I used a TL071 opamp set up as an amplifier as shown on the bottom right here http://www.muzique.com/lab/boost.htm. The problem is that while it boosts the level, it also clips rather hard, which I don't want. I just want a simple, clean level increase. I am using a 12V supply (I think it is rated for a max 18V).

I was wondering if I am overloading the input, so I used the old meter to see what kind of max voltages I am getting and I get a max of about .5V peak to peak, which shouldn't be near enough to clip the opamps input. I guess I could be clipping the inverting input, but the volume increase from unity on up is gradual enough that I wouldn't have thought it.

My question then is what might be the issue. Also, if anyone has a better suggestion as to a solid state gain recovery scheme, I am willing to give it a shot. Thanks.

It is not about what you put into it but about what voltage you need to get out of it. Using an opamp design you should run at +-15V or 30V with a bias. That is showing a bias. Also with a bias at 4.5V you would be lucky to get 4V rms clean out of the circuit. You could kick that up to 30V with 15V bias. Show me your schematic on the gain stage since there are missing components here.

[Structo]

Ripthorn,

Here is a way to get 30v from your amp power supply.

Just connect the input to the node right after your first filter cap.

This will power the opamp cleanly.

The 25K and 20K resistors are 5 watt.

[drz400]

You can also use a 12V to +-15 DC to DC converter if your 12V is already regulated, Piece of cake. No resistors

[Andy Le Blanc]

I got into an old Alamo that had a transistor reverb recovery powered from
the cathode bias voltage, it wasn't too far off from the verb recovery in
a
Sunn. One transistor, real simple , high impedance, probably work with 10
to 20 something volts. Check out the sunn scheme at schematic heaven.

[Ripthorn]

I have been working on this some more, and attached is a schem of what i have right now. It is powered on +/- 15V at the moment, though I have ordered an opamp that can run on +/-22V (max) so that I can see what else I can squeeze out of it. Unfortunately, I don't have an oscilloscope to check things out, so tuning the gain after the distortion tonestack is tough, since I can't tell when the opamp is clipping and when it is just the input signal. It seems like +/-15V ought to get me pretty high, but it is still several dB lower than the bypassed signal. Thanks for the suggestions all, I will keep at this.

[img614]http://i652.photobucket.com/albums/uu24 ... covery.png[/img]

[drz400]

You should add a cap to limit the low frequency amplification.
Add a cap in the feedback to limit high frequency amplification.
Small value cap at the + input to ground to help with any RF.
Series resistor of about 2K at + input to prevent any static shock damage unless there will be no connections to the outside world.
Need a resistor going to ground on the output unless it is terminated in the circuit it is feeding.
Power supply decoupling
What impedance are you driving would determine the output coupling cap size.
5532 would suit the purpose just fine.

This is a great bible if you can find one
http://www.abebooks.com/servlet/BookDet ... 0672215586

[Ripthorn]

Thanks for the feedback drz400. What do you mean by power supply decoupling? I just got a 5532 that I am going to stick in there and see what I can get out of it.