• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

How do I properly reduce the gain of my preamp?

Hopefully after the coupling cap, i.e. grounded the grid of the second tube.


If those statements are both true, then the noise is created in the first stage or comes from the supply. So, for a given listening level, what you propose is to cut down the signal at the output of the stage, then increase the input signal proportionally, until the signal swamps the noise. Somehow I find that sub-optimal in a $5k piece of equipment🙂

Here is what I find absurd about this thread. Presumably the reason someone buys a $5k preamp is a sonic quality unobtainable at a more reasonable price. A brilliant design in other words. So now we try to re-design the circuit instead of fixing it. The only logical conclusion is that we all think the original designer is an idiot. Is that reasonable?

As Jean-Paul said, time to step back and re-examine what we're trying to do .

Preamp Noise Issue

Here has all the info about the noise I got from the preamp. TBH, I have almost everything I can, but reducing the gain seems to be the easiest way that can end my pain.
 
It's likely that preamp's gain is too high for his system. He may even have 100dB+ peaky speakers.
But he wants to use that preamp, somehow with less noise. That's his choice, but it would not be mine.

My speakers are 93dB (Cube Audio F8 Nenuphar), but you are right, my system does not need this much gain. I use preamp is more about having the "black magic". Indeed, it made the sound better by having this preamp, but also introduced the noise...
 
Then I strongly recommend the method in post #35 as the most likely best solution for your situation.
Start with the mentioned values and see.

There is a related alternative method, but it possibly could affect the higher frequencies. The post #35
option will not change the sound of the preamp at all, unless you reduce the gain so much that the
volume control must be turned up very high, which could overload the input tube's grid.
 
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Here is what I find absurd about this thread. Presumably the reason someone buys a $5k preamp is a sonic quality unobtainable at a more reasonable price. A brilliant design in other words. So now we try to re-design the circuit instead of fixing it. The only logical conclusion is that we all think the original designer is an idiot. Is that reasonable?

As Jean-Paul said, time to step back and re-examine what we're trying to do .

This is what one sees very often with the tube crowd (consumer side). People want to do something and buy a nice large rim and tire. They connect this extra rim and tire to their car and then they have 5 wheels. Then they find out things don't work out and try to change the engine, the seats, the spark plugs and so forth. Maybe the fifth wheel was not such a good enhancement? Still fellow car enthusiasts will advise to change the fabric of the seats, change tires again and to repaint the car. Then one thinks of changing the bolts of the fifth wheel....

As you can see one has to pull very hard to get information about the chain which really are the easiest things to answer (and should be mentioned in the first post). This is an indicator that desired result is a question of belief (add some tube magic whatever that may be) not a scientifical approach. Any person that adds too much gain and noise and god knows what would first go back to a straight wire to be able to evaluate. Or a simple selector switch, volume potentiometer and a OPA1656 opamp buffer in a box with some RCA connectors. With just 2 resistors one can determine what gain one needs (answer: 0) and then find/buy/build a suitable device that fits in the chain.

So:

1. one has sources. Which sources, how many and what output levels and impedances?
2. one has a power amplifier. What gain, sensitivity and what input impedance?
3. are there any non standard or special issues that have to be taken into account as they may influence the end results in a negative way?

Issue: source selection and volume control. Find a way to select sources and control volume the optimal way, respecting the issues found under 3. Try to find 3 options and describe their good things and their bad things. Choose the option that has most good points.
 
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This is what one sees very often with the tube crowd (consumer side). People want to do something and buy a nice large rim and tire. They connect this extra rim and tire to their car and then they have 5 wheels. Then they find out things don't work out and try to change the engine, the seats, the spark plugs and so forth. Maybe the fifth wheel was not such a good enhancement? Still fellow car enthusiasts will advise to change the fabric of the seats, change tires again and to repaint the car. Then one thinks of changing the bolts of the fifth wheel....

As you can see one has to pull very hard to get information about the chain which really are the easiest things to answer (and should be mentioned in the first post). This is an indicator that desired result is a question of belief (add some tube magic whatever that may be) not a scientifical approach. Any person that adds too much gain and noise and god knows what would first go back to a straight wire to be able to evaluate. Or a simple selector switch, volume potentiometer and a OPA1656 opamp buffer in a box with some RCA connectors. With just 2 resistors one can determine what gain one needs (answer: 0) and then find/buy/build a suitable device that fits in the chain.

So:

1. one has sources. Which sources, how many and what output levels and impedances?
2. one has a power amplifier. What gain, sensitivity and what input impedance?
3. are there any non standard or special issues that have to be taken into account as they may influence the end results in a negative way?

Issue: source selection and volume control. Find a way to select sources and control volume the optimal way, respecting the issues found under 3. Try to find 3 options and describe their good things and their bad things. Choose the option that has most good points.

I like your thinking and logic.

I too have a preamp with WAY too much gain 26dB and I am in nostalgia mode I think, also trying to cut down noise and volume wheel sensitivity. So far I attempted to replace it with more temperate preamps, borrowed from friends for a free trial. A £4k Luxman, a £4k Tron, and old Naim, and I also tried to attenuate in the incoming signal, and in the connection between the pre-power with a voltage divider made from quality resistors.

Sadly that damn 35 year old preamp has some magic that is just dramatically missing in all the attempts so far. HOWEVER, I have never actually got it completely out of the way = no gain. This might be interesting, the speakers are 99 dB/W, and the power amp is as I understand it 2V for full output - it's a 12W 211 valve amp, so it does not need any more gain. (except in phono, which I am not using atm)

I had logically inferred that the valve pre amp was adding a dynamic contrast associated with it's ridiculous sensitivity that created the immediacy and realness it offered and the valves helped the tonal richness I like.......BUT maybe it's time to look at a passive again. Past experience with passives were they sounded flat and lifeless, however I never really experimented beyond 1 example....
 
Your preamp should be tempered with regards to gain as you have (like more than 75% of tube preamps) way too much gain. Somewhere in the eighties 2Veff came around but the tube guys were listening to their records and missed the news 😀 Seriously, it is one of the most found issues and apparently people try to marry stuff that really better both should stay single. Why tube guys today are fond of high gain is a mystery. It probably comes from the era gain was much needed like with analog record players, radios, microphones and such. Today anyone has 2Veff sources that have low output impedance and can drive quite difficult loads. More capable than most tube preamps can anyway. From a technical point of view it always leads to irritation as given reasons are illogical like sound color, tube magic etc. while one can easily see it distorts. If one adds too much gain one adds too much gain. Simple. Building a chain where parts amplify, attenuate and amplify again is not a HiFi approach IMO. Why someone would accept volume control that gives full output at 1 is incomprehensible. My deliberate experiments with devices that needed to be at 12 o'clock for normal volume always led to dissatisfaction too.

Regarding passives: when do we call it a passive? A power amplifier with volume control and source selection is called an integrated amplifier. In reality there is no buffer of active part after the volume control in most cases. Distances are short and things seem to work OK for a very long time now. What I would call a passive is a separate device without any buffering driving a cable to a power amplifier. This setup never satisfied me much but YMMV.

In this thread subject the silly thing is that a self created issue that costs 5500$ is probably the wrong choice as no things technical were considered beforehand. It reminds me of a therapist I once visited as I had severe back aches. She started the treatment and was mumbling and when I asked what she did she answered:"I don't know, I always just do something" 🙂
 
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ok - I think I get the logic 🙂

So as an experiment (and I can worry about phono again if I ever get my turntable plugged back in 🙂

So for the 'integrated amplifier' approach, I could make a trial with a small box to hold things together so wires aren't dangling around containing a potentiometer in the signal path from the source to the power amp. (There's no room to get a potentiometer into the Power amp easily with what exists currently.)

Would this tell me all I need to know on the value of my vintage pre-amp?

🙂

I have a 50k pot, and the input impedance of the power amp is 100k ?
 
For the trial the 50 kOhm is OK if it is logarythmic. You can make the trial device as you suggested but make sure to check sources for DC at their outputs (just as a precaution) and check the power amplifier to have an input capacitor. Any modern source drives even 10 kOhm loads with 2 fingers in the nose as we say here. If you keep it passive cables to the power amplifier are best kept short. If you like comfort and somewhat longer cables an OPA1656 as buffer is your friend. Just for a short trial of course as its SQ is addictive.

It would not hurt to include a high cut off filter before the volume control.
 
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No it is not necessary. It would result in half the sources signal too. Comically this is what you could do in the existing preamp (no, don't) 🙂

Please read this:

Help in choosing a potentiometer as a "Passive preamplifier"

I just reread your power amplifier to be a 12W 211 valve amp with 2V sensitivity for full output which is uncommon for a tube amplifier. My guess is that you will need some gain but you will learn that with the trial. As said a device with opamps with gain that can be changed with 2 resistors is a good way to find out what gain is exactly needed if one likes the empirical method.
 
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tube amps, the common cathode triode amp can be designed for maximum gain or maximum output swing and anything in between.....

download an RCA receiving tube manual and go the the rc coupled line stages somewhere at the back and you will see what i am talking about...

in those tables, you will find values of B+, and resistor values with each section of a tube type, yes even for pentodes..

basic gain formula is A = gm x Rl, gm is tube transconductance and is cathode current dependent, Rl is the tube plate load which is basically rp//Rl//next stage grid resistance.... rp is plate voltage dependent, the higher the plate voltage, the higher plate resistance becomes....

so that from these simple relations you should be able to figure out voltage gains..
 
Ok - so I threw together a VERY rough test. I took some solid core hook up wire and twisted around the output tags of my old preamp potentiometer. I took a single earth from the pot and then soldered in some naked phono plugs for left and right channel. The pot in the preamp is a good quality 100k (TKD) and I did not shield anything.

So the input from the DAC went into the pre amp and the pot in the preamp fed the power amp.

It picked up fizz/interference in this naked form, and was sensitive to touch and proximity.

However it showed some interesting results

Slight but not crazy loss of dynamics
more clean/less hash
enough volume for what I need
some loss of bottom end

I guessed 2v for full output, as I don't know the spec - I might be able to find out.
 
Here is what I find absurd about this thread. Presumably the reason someone buys a $5k preamp is a sonic quality unobtainable at a more reasonable price. A brilliant design in other words. So now we try to re-design the circuit instead of fixing it. The only logical conclusion is that we all think the original designer is an idiot. Is that reasonable?

Brilliant?? The circuit shown at the beginning of this thread? I disagree. It might be only good to drive a booster (i.e. a very insensitive power amp), assuming no noise issues....

If yours are conclusions I can bet that if one buys a more humble, old, second-hand CJ PV-10A (which is basically what I suggested, a common cathode + follower but with the lower gain, often insulted, 12AU7) will significantly improve sonic quality at a fraction of the cost. Higher cost is no guarantee!

Many things Audio Note UK have done in most recent years are questionable and yet they are more and more expensive. Nothing to do with the real Audio Note of Kondo San. So much so that his son founded Kondo Japan to preserve that heritage and maybe go a bit further.....
 
I think dgta means the AN device as a whole which he actually writes.

No extra/superfluous device can improve sound quality compared to a straight wire (if there is no gain/impedance mismatch of sources and power amplifier). Tonescout is the one that dared to do the trial. He probably will be pleasantly surprised when he builds it in a case with all basic rules. If the sources are not OK and/or the power amplifier is not OK one could try to improve those if necessary, their presumed negative qualities can not be solved by adding an extra device "improving" what they mess up.

Since the OP has not mentioned brand/type of power amplifier and gain/impedance and OP admitting that he does not need gain at all we may assume that in this case the AN is completely unfit for the job and even detrimental but for the added tube magic like absurd high gain and noise. Considering the high price of the AN one better not messes around in a proven design and tries other ways to make it happen.
 
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Brilliant?? The circuit shown at the beginning of this thread? I disagree...

You need to read my post more carefully. I did not state an opinion on the design or designer. I was simply reasoning through the logic of what we're doing here.

As to AN, I was not at all familiar with them, so I later looked at a few schematics of their amps at random. Just out of curiosity. My opinion now is that they design and make bizzarre circuits with the apparent purpose of confusing the innocent and baffling the unaware. In one amp, for example, the line stage consists of two 6SN7 sections in parallel. However, the two sections are not in one bottle. They are in two different bottles, with the other half of each bottle shorted (a, k & g) to ground. Let's just say I find it...unusual.
 
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by removing the cathode bypass of the first stage, you can reduce the gain, by decreasing the plate load of resistor you can also reduce the gain.....you can do any of these or combinations..

by reducing R9/R10 from 1meg to 100k will also reduce the overall gain..
 
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Here is what I find absurd about this thread. Presumably the reason someone buys a $5k preamp is a sonic quality unobtainable at a more reasonable price.

Even more disturbing, when you go to the web page where this preamp resides, there isn't even a specification sheet where you can look at things like gain, noise, etc.😱 So you spend a lot of money not knowing what you're going to get????

jeff
 
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