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Hamstein 13th May 2012 12:05 AM

Whats wrong with this circuit?
I recently acquired a Dared MP-15 hi fi amp, it is a pleasing looking shiny thing! Using two KT 94's and four '6N3's'.
It sounds pretty good with inefficient speakers, like my little Wharfdale diamond 4's.
Unfortunately with my Lowther EX3's in Hedlund Horns, it is too bass heavy, and too powerfull!
I have 'triode'd' it by connecting R22 direct from pin 4 to pin 3 of the KT, -lowers the output!
If I disconnect the feedback loop from the 8 ohm term. Gain increases! But no noticeable tone effect!
The circuit looks overly complicated to me! Any suggestions for modifications!
I need lower gain but a nice clean throughput!

I emailed Dared, and they were nice enough to promptly send me the schematics. Only a couple of differences on my amp!

Here is the main 'per side' circuit, on the actual board are the additions R42 and C10, which I have scrawled in!

DavesNotHere 13th May 2012 12:49 AM

There is nothing wrong with the amp. The amplifier is amplifying beyond the bandwidth of your speaker.

There are two major ways you can tweak the bandwidth. The first one is changing the coupling caps to a lower value. This is the most common way. It was done purposely in the radio era days to prevent speaker flopping when the tuner was tuned on/off station setting. The second way of narrowing response is to lower the capacitance value of cathode bypass capacitors. this creates a higher impedance signal ground for the lower frequencies. Sometimes total removal of the bypass capacitor is needed. Keep in mind that this effects the amplification factor, but what you might gain is better balance.

There is the feedback too, but that really effects the high frequency attenuation. tweaking this part of the circuit effects the mid band frequencies (presence control is a special adustible neg feedback)

I would reduce coupling cap to .1 uf and if it still not to your liking, remove 100 uf bypass cap.

but try both ways, (bypass cap first) and see how you like it since all you have to do it lift one side of the cathode bypass cap.

Hamstein 13th May 2012 01:09 AM

Thankyou for your advice, which coupling capacitor should I change? Or both? I don't really want to limit the frequencies passing through, just get a more linear response, at lower gain levels.

Hamstein 13th May 2012 01:25 AM

I am very familiar with the design and modifying of guitar amps, but Hi Fi is new to me, the same rules do not apply! :confused:

DavesNotHere 13th May 2012 01:31 AM

have you tried lifting the 100uf first? as that is more effective as no bypass capacitor would give you true linear response.

as far as the coupling caps, the first one would all you'll need to change. The coupling caps are going to give you a 5-20% change. But may induce an undesirable phase shift.

DavesNotHere 13th May 2012 01:40 AM


Originally Posted by Hamstein (
I am very familiar with the design and modifying of guitar amps, but Hi Fi is new to me, the same rules do not apply! :confused:

they do, but the don't, as guitar amps are non-linear stages by design.

the audiophile design falls in the category of the sudio mic pre amp and comp designing. The I/O in pro is balanced and the audiophile/consumer is typically unbalanced.

Hamstein 13th May 2012 01:40 AM

I shall try that. I think i need to lower the gain at the power amp side though,( ie just before the '88 ( sorry kt94!);)

DF96 13th May 2012 02:21 PM

If you need less gain why not just turn down the volume control - that is what it does!

Converting to triode mode by moving R22 will reduce open loop gain and output impedance, but make less change to closed loop gain and output impedance because of the feedback.

You could reduce gain by reducing the 10k feedback resistor (R40?), but this might create instability problems at LF or HF.

Removing C12 will lower gain but increase output impedance. This may, or may not, match your speakers better. It would depend on the extent to which they rely on electrical damping of the bass resonance. You could get more bass!

Bear in mind that this is not a guitar amp to be 'voiced' but a sound reproducer - it is designed to output a faithful copy of the input. It won't do this perfectly with such a simple circuit, but it will try. Feedback will affect all frequencies - 'presence' controls on a guitar amp are quite different as they are specifically designed to modify the sound.

Hamstein 13th May 2012 10:38 PM

Thanks, a few things to try out there!

Using my Lowthers I have to have the volume pot turned so low that it is inaccurate between the channels.

I could attenuate the input just after the pot, but I would prefer that I was amplifying a larger input by a small amount rather than amplifying a small input by a large amount if you see what I mean! - Better for s/n ratio.
Hence why I would like to lower the actual gain of the amplifier circuit.

bear 14th May 2012 05:33 AM

One might consider merely putting a resistive pad (attenuator - two resistors) at the input of the amp to increase the voltage required to drive it to full power?

Your Lowthers are ~100dB/1w, so you need not that much power to drive them, and not that much gain... I think that you can loose 10dB in the front end and not notice much problem in the S/N department...

Or dork the resistors on the second stage (hard to read on my screen) so that it is lower in gain. More like a unity gain stage...

Oh, you could also attenuate the signal between the first stage and the second, that might be a good idea too... easy enough. No S/N loss like that... you have too much signal coming from there now! :D

You can either mod the gain, or mod the level. Modding the level is easier and not likely to alter the stability any.

Check ur HF response too... see if you can pass a sinewave flat out to beyond 20kHz, or if it is falling off by 5kHz or something like that?

What other amps have you used on the Lowthers?
Lowthers rarely are accused of too much bass...


But the question is why does it have too much bass?
Is it because it has too *little* highs?
A scope will tell us.

Check the square wave at 1kHz. See if it looks square on the top, and without ringing, not too over damped on the leading edge.

Post it here if you like - make sure the input square wave is square, post that too.

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