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Old 3rd July 2011, 05:44 PM   #1081
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Quote:
Originally Posted by andyjevans View Post
Coolzero - is that a 5uF output capacitor? Is this going straight into a tube amp stage? Is it so close to it so leads can be very short? If that is true, and you have say 270k to 470K total input resistance on the amp (your two resistors in parallel) then a smaller capacitor would give you much better sound, because you could use a better quality capacitor. I'd use Russian 0.1uF Teflon FT-2 or FT-3 in whatever numbers give you a good enough bass response. Parallel for 0.2, 0.3 etc.

I have short leads to my amp and I use a single Teflon 0.1uF FT-2 cap. Inevitably capacitors will have some effect on the sound so this part must be optimised.

If you have a transistor amp that's another story, but surely you don't?

Andy
I have a 100K input impedance transistor amp, what about output cap?
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Old 3rd July 2011, 06:07 PM   #1082
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Thanks Thomas for your valuable advice. I usually turn the pre on for a while to avoid static charge before I turn on my power amp. So practically I do not get the loud pop when operate.

About bias voltage; the issue I have with higher voltage is that I am getting bit of noise when I use anything less than with the given plat voltage. Basically the higher plate current making my pre to produce little bit of noise and making the speaker cones to wobble even there is no signal from the input. I tried all the option but reducing the plate current was the only way I could observe to reduce this noise. Other-way around is to reduce the output cap value but then the bass is going for a toss. Couldn't understand why exactly the noise comes. I am feeding the pre to a power amp directly with an input impedance of 33K I believe. I assume it is bit lower for 26. BTW, I removed the grid stopper after reading one of your post to Andy.

About the pot; what do you think suitable for this application? Appreciate any of your expertise to fine tune my pre.
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Old 3rd July 2011, 06:07 PM   #1083
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You can either calculate the output cap and choose a value by equations, or you can listen and choose a value that gives you good enough bass in your own system - more capacitance gives you more bass. If you have a home theatre system with a subwoofer you may need a large capacitor. But if you have a system that is relatively light on bass - like a full range speaker with no subwoofer (which I have for instance) - then you might be able to use a lower value. Depends how low your bass goes. This is assuming that you build your own system and can choose the values that suit it rather than "book" values. In the bad old days the output capacitors were electrolytic. Yuk.

Andy

Last edited by andyjevans; 3rd July 2011 at 06:09 PM.
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Old 3rd July 2011, 06:18 PM   #1084
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Quote:
Originally Posted by andyjevans View Post
Coolzero - is that a 5uF output capacitor? Is this going straight into a tube amp stage? Is it so close to it so leads can be very short? If that is true, and you have say 270k to 470K total input resistance on the amp (your two resistors in parallel) then a smaller capacitor would give you much better sound, because you could use a better quality capacitor. I'd use Russian 0.1uF Teflon FT-2 or FT-3 in whatever numbers give you a good enough bass response. Parallel for 0.2, 0.3 etc.

I have short leads to my amp and I use a single Teflon 0.1uF FT-2 cap. Inevitably capacitors will have some effect on the sound so this part must be optimised.

If you have a transistor amp that's another story, but surely you don't?

Andy
Andy,

I am feeding this to a transistor amp with 33K input impedance. All the caps I used are Mundorff. BTW, these values were suggested by Kevin taking consideration of the parameters I have given. Actually it was calculated for an amp I used before with 20K input impedance. However I tried with different values greater and lower than this with good quality caps from Mundorff to Auricap and Russian PIO but the current value gives the best response for my ears. Bass is very tuneful and tight. Treble is better but I think still there is a room for an improvement there.
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Old 3rd July 2011, 06:20 PM   #1085
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Hi coolzero,

you should find and fix the cause of the noise rather than adjusting other parameters which certainly have nothing directly to do with it. The noise can be caused by lots of things. You should try a systematic approach to identify it.

Is it on both channels or just on one? If on both, it is probably an issue of the power supply (assuming channels share the PSU), maybe motorboating. In that case remove one tube. Is the noise still on the other channel?

The 26 should run at higher currents without any noise.

I'd use a 50k pot, should be better. Of course if you want to do a radical change and if your gain is enough, go for a step down transformer instead of the plate choke and use a TVC at the output. This will also drive your 33k input impedance of the power amp much better.

Removing the resistor at the output is bad practice. This has nothing to do with warm up. You can use a higher value though, like 1M. But again with a good design and sensible op point this should not make a difference.

Try the above things to identify the source of the noise, depending on the results of the test we can check further.

Best regards

Thomas
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Old 3rd July 2011, 06:24 PM   #1086
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Quote:
Originally Posted by andyjevans View Post
You can either calculate the output cap and choose a value by equations, or you can listen and choose a value that gives you good enough bass in your own system - more capacitance gives you more bass. If you have a home theatre system with a subwoofer you may need a large capacitor. But if you have a system that is relatively light on bass - like a full range speaker with no subwoofer (which I have for instance) - then you might be able to use a lower value. Depends how low your bass goes. This is assuming that you build your own system and can choose the values that suit it rather than "book" values. In the bad old days the output capacitors were electrolytic. Yuk.

Andy
I have floor stand speaker with a relatively large cabinet and goes down to 35Hz as per the original specs.
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Old 3rd July 2011, 06:26 PM   #1087
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Hi!

Regarding output caps, with such low input impedances you need several uF capacitance. There are plenty available which sound good, I like to use Sangamo paper in oil caps. I'm not so fond of 'boutique caps' like Mundorf.

But as mentioned before I don't think the 26 plate loaded is a good choice to drive low input impedance amps. It has a Zout of 7kOhm. Rule of thumb is that Zout of the preamp should be one tenth or less of the input impedance it drives.

In general I prefer lower Zout preamps even with tube power amps. Makes the preamp to react much less to loads (cable, etc) on it's output.

Best regards

Thomas
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Old 3rd July 2011, 06:29 PM   #1088
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Hi again,

I recommend to first fix the issues of the preamp, before you start to fiddle with the output cap or volume pot. First it should work flawlessy at higher current and should run stable. Once that is achieved, it makes sense to try to optimize other things.

Thomas
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Old 3rd July 2011, 06:48 PM   #1089
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Thomas,

There were few hints given by Rod as well to narrow down the cause of this noise. Most probably it could be the SMPS I used for bias supply which I am planning to replace. I will post the result as it progress. As well I will apply your suggestions in due course. BTW, do you have any recommendations for the step down and TVC?
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Old 3rd July 2011, 06:53 PM   #1090
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Quote:
Originally Posted by Vinylsavor View Post
Hi!

Regarding output caps, with such low input impedances you need several uF capacitance. There are plenty available which sound good, I like to use Sangamo paper in oil caps. I'm not so fond of 'boutique caps' like Mundorf.

But as mentioned before I don't think the 26 plate loaded is a good choice to drive low input impedance amps. It has a Zout of 7kOhm. Rule of thumb is that Zout of the preamp should be one tenth or less of the input impedance it drives.

In general I prefer lower Zout preamps even with tube power amps. Makes the preamp to react much less to loads (cable, etc) on it's output.

Best regards

Thomas
If you could suggest, I would like to try any other option which applicable for my setup, gives the same sonic qualities as 26. Thanks BTW for all your advice.
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