Help with Keith Kirby's DIY Tube Preamp with dual 6H9C/6SL7 and one 53YGT/5AR4.

[Eli Duttman]

"Padding" resistors between the RCA I/Ps and source selector switch are an alternative to fussing with loop NFB, in dealing with a "hair-trigger" volume control. Metal film parts hold S/N degradation within acceptable limits and "padding" resistors allow custom control of the listening level for each of several signal sources, with their potentially differing drive levels.

Having options about which way to "jump" is a good thing.

 

[kodabmx]

Yes. I was thinking that if Kingsley needs less gain than this circuit, an easy idea would be to add a fine volume adjustment. Or just a gain switch. One side on a voltage divider.

 

[kodabmx]

Kingsley bought two 6SN7 GE tubes. One looks new, the other looks very used. In this circuit though, they are within 1V of OP. Still, to burn in this design, I'm using some 6N8S from the used box.

 

[audiobasic2000]

Hi Eli and kodabmx:

First off - Apologies for not chiming in earlier as I was at playing badminton/getting some exercise and only got back a few minutes ago.

The input padding resistors sound like a good idea versus NFB. Does each input require a different resistive value or the same value for all inputs?

As for needed gain, can't really offer much input - what's the general consensus? As far as I have read/seen, most amps have input sensitivity that ranges from .5V-2Vs. E.g., A Jolida SJ502 has an input sensitivity of 600mV; the Muse 100 - .890V, an Acoustat TNT-200 - 1V.

kodabmx, may I ask what you are referring to as 'fine volume adjustment'? And as for the gain switch, are you referring to one mounted in/on the chassis which would require drilling a hole(s) for mounting it?

One of the two 6SN7s I bought along was used in another component (forgot what it was) although they were bought at the same time from a member of Canuck Audio Mart. Not sure if you are referring visually or actual testing, kodabmx.

Thank again.

Sincerely,
Kingsley.

 

[Eli Duttman]

A single padding resistor in each channel can be employed between the O/Ps of the source selector and volume control. However, the method lacks flexibility. A phono preamp and a CDP will likely exhibit different signal levels, which requires the listener to reset the unit after changing sources. Individual padding resistors between the I/P RCA jacks and the source selector can be sized to bring disparate signal levels into alignment.

 

[kodabmx]

One voltage divider is all that is needed. After the input selector, before the pot. This could be made switchable if needed. Hell, even a "course and fine" setup would work. Two volume controls, one on the back as a trim.

I also have a pair of these which could be used at the input to the power amplifier. Good for a 6db drop. Fmod Style Fixed Value Inline Attenuators

 

[audiobasic2000]

Hi kodabmx:


Not gonna use inline attenuators (have seen and read about them) if at all possible. (Only Scott Endler or EVS Ultimate Attenuators have minimal effect on sound quality but then why waste that much dough when the circuit can be attacked to correct the gain.)



The coarse and fine setup is a no go too as it requires drilling of the chassis. (Now there's two volume controls in the path which kind of defeats the purpose of having a volume control.)



Voltage divider with a switch would/might be more logical but may also entail drilling into the chassis to mount the switch.


Thanks, Eli, for the clarification of the padding resistors.


Thanks.


Sincerely,
Kingsley.

 

[kodabmx]

Course and fine have the same effect as a resistive divider, just that it's adjustable. It can be mounted inside the chassis (or on the rear for ease of use) But in reality, it's a matter of "set it and forget it" in which case fixed resistances make the most sense, and resistors only cost a few cents each, pots do not.

In any case, I don't think you'll need them. I think this amp has a standard amount of gain.

I also think it's better to have more gain than you need then not have enough.

 

[audiobasic2000]

No argument/disagreement from me regarding your last post, kodabmx.

 

[Eli Duttman]

I also think it's better to have more gain than you need then not have enough.

True enough, but not so much net gain as to observe a "hair-trigger" volume control.

JMO, sort the line stage out, with a CDP as the signal source. If necessary due to "hair-trigger", pad the I/P down. Sort any other sources out, at some later time.

 

[kodabmx]

I don't have a CD player, I rip them using the drive in the computer when I need to. On a 1V source like the DAC, the volume control is "normal", with a 300mV input from the television, a "normal" listening level requires almost 1/2 volume.