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Old 14th May 2012, 04:53 AM   #11
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Might be fun just to remove the first stage. Just lift C4 and run the volume control straight into the paralleled second stage. With the output trioded and high sensitivity speakers, may be just enough. Easy to try and easy to restore if it isn't enough.

All good fortune,
Chris
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Old 14th May 2012, 10:33 AM   #12
DF96 is offline DF96  England
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This is designed to be a high gain circuit (voltage gain around 50). You could increase feedback but you might need to reduce open loop gain to compensate otherwise you may get oscillation. Reducing open loop gain is not easy, as the circuit has been designed for high gain.

One thing to try: reduce R8 to 1k, and increase R6 to 390. This will increase feedback, and reduce the gain of the first stage (although by a smaller ratio). Whether this works will depend on how close to instability the original circuit is.

If you get HF oscillation or peaking try increasing the value of C8. If you get LF oscillation or peaking try reducing the value of C10 or omitting C12.

To be honest, an input attenuator wil be simpler!
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Old 14th May 2012, 05:27 PM   #13
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Quote:
Originally Posted by DF96 View Post
...To be honest, an input attenuator wil be simpler!
I wonder what you think the basic problem is here? I assume that turning down the volume has not had a satisfactory effect, and this is related to the characteristics of the speaker.

It's a while since I looked, but is it not common for SETs to have a high-pass filter, often two-stage, at the input? The idea I think is to stop unmanageable low frequencies before they produce harmonics, IM, output stage distortion, or a range of transformer-related effects.

Considering this amp has plenty of feedback, then its output impedance is probably low enough to cope with a widely-varying load impedance, so wouldn't explain the perceived problem. Its frequency response is also extended by the feedback so, at least for small signals, that isn't the problem either.

For larger signals, if the load impedance falls very low, distortion can rise steeply just as the proportion of feedback left to deal with it is dwindling. The harmonics produced by the output stage are efficiently reproduced by the speakers, and peceived as bass emphasis.

Perhaps it would be good to find out what the open-loop bass response of the output stage is, and use an input filter to limit the amp thereabouts.

The idea of disconnecting the feedback and abandoning the first stage could be a useful diagnostic experiment, even if it doesn't immediately give satisfactory results.

From what I have seen of SETs and single-driver speakers, it's a delicate illusion that requires a lot of messing about to get right.
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Old 14th May 2012, 10:58 PM   #14
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Thanks folks, and please keep up the suggestions! I shall probably try most of them out at some point. I did get this amp with a view to playing around with it's internals!
Unfortunately my old oscilloscope has breathed it's last, so I'll not be able to post any traces of my investigations as yet.
Lately I have been running the horns with an old Nad amp, which has a low volume button, very useful for the purpose. (everything packed away at the moment while I redecorate the front room though) - They sound fine using that.

Might be a little while before I have another tinker with the amp, as I slipped and fell down the stairs today, and am feeling a bit battered and sorry for myself!
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