Tame my pre amp

Hi,

This is unlikely to have much effect. The circuit uses negative feedback to set it's closed loop gain of around 20db (or so) and two stages in series are used inside the feedback loop (in fact, there is additional positive feedback in the second stage to maximise open loop gain) so even the 12AU7 will have with both sections in series an open loop gain of approximatly 20 * 20 (12AU7 Mu = 20), so an open loop gaon of around 400 for a closed loop gain of 10 vs the open loop gain of 10000 (appx.) when using the 12AX7 - changing the valves will make no appreciable difference to the closed loop gain aseven the lowest gain valve has loads more gain open loop than with the feedback loop closed.

Which again will make little impact in this amplifier circuit, if any.

That was already if you look closely and the results where not liked much according to T-Bone.

So, while you input may be generically relevant (not always though), it is completely off base here.

Ciao T

 

I could change the tubes but it is no longer on this earth, its entrails now rest in the surplus component box and Im am well underway with the 5687 design, another couple of hours and it should breath for the first time (or blow up).

I dont like it when people say that something I do is brave! makes me a tad nervous. The schem looks quite simple and everything looks right with what Ive done.

Ive actually got some Jensen 0.1uF caps which I was going to use but I also have some 0.47uf PIO's that I could replace them with if needs be. I have used a BG on the cathode.

The power amp has a 1M resistor to ground on the input but that could easily be changed if thats what you recommend??

 

Hi,

With 1M you are fine, 0.1uF will do splendidly.

Ciao T

 

Well the beast is up and running and I must admit even in these early stages it does sound pretty good. Just one or two probs, there is a 'hiss' present all the time, nothing major but I would like to banish it. The Gain must be pretty dam high coz I barely need to move the volume to get loud levels, I darent even try it past 12 O'clock (could these two probs be associated??) I do have some microphony when I tap the chassis, Im presume this is coming from the two 0.1uF caps coz I aint fixed them yet (didnt know if Id be swapping them.

Apart from that all is pretty dam fine, any ideas Mr L?

 

Hi,

The high gain and the hiss are linked. Kondo Amplifiers all have 100K input level controls, you would set these to match the Preamp for good volume control range and best sound. This is a common approach in Japan, also found with Shindo.

If you want less gain connect a 220k Resistor from the output (post coupling capacitor) to the grid of the Valve and 100K either between volume control wiper and grid or in series ahead of the volume control.

You have now added a lot of local feedback and gain will be much lower, as will be noise, but it will not be a "Kondo" circuit anymore...

Ciao T

 

Come on T, which one, Im prepared to try either!

 

Actually, by level controls I am presuming you meen an attenuator or 'pot?

 

Hi Tbone,

What power amp are you using? The type of linestage hiss you describe is typical of a high gain valve linestage into an amp with high i/p sensitivity. The Kondo power amps do have input pots but these too are just acting as a potential divider with an effect on sound/dynamic range. It is why people look at output transformers to reduce the stage gain and o/p impedance or, as I'm doing, look at the low gain small power triodes. It's fun trying to get them quiet though.

(some even go for TVC's!)

Thorsten's feedback suggestion may not be as catastrophic as you might expect and at least would make the pre usable and still better sounding than what you had originally.

depending on what power amp it was I'd be looking to reduce the gain of that!

 

Hi,

Yup.

Ciao T

 

Did you do any further modifications to make the linestage quieter?

rgs
Murray

 

Sorry chaps, had no broadband for a few days. Thanks for the input Murray, Ive not done anything else except install a stepped attenuator on the power amp (which started life as a cheapo dynavox and has since been totally rebuilt). Funnily enough, it needs very little attenuation to get it silent, in fact I measured about 10k to be precise. I've left it at around 50k (when I do the measurements) coz that gives me a usable range on the pre amp.

Apart from that Im well happy, it sounds great and is much better than anything I've had before. My one and only gripe is the microphony of the tubes. Using tube dampers has solved about 90% of this but if tap the tubes (which, of course, I tend not to do) and still get a rining throught the speakers. I'm presuming this is the nature of the tube??

Anyway, all is well really!

 

Its normal....................!

 

Hey T,

Just a quick question:

What sort of voltage would you expect to find at B+ on the Kondo psu?? Im away from home and playing with PSUD would like to know if Im on the right track!

 

Hi,

250ish

Ciao T