New DISPRE preamp, successor to previous popular version

[Bonsai]

Ok - thanks - its clear.

 

[hags]

Volume pot.

Does anyone see any reason why you could not use a 25K volume pot. with this circuit instead of the recommended 10K Alps?

 

[PMA]

You can use it, but increase R3 and R29 from 24k to 47k.

Regards,

 

[hags]

Pavel,
Thanks. Will increasing the value of R3 and R29 have any deleterious effects?

 

[PMA]

No. The main reason why I prefer 10k is that lower resistance of the volume pot results in less change of frequency response as a function of pot angle, and also the input stage is driven from lower impedance, which I do prefer as well. But, 25k is still acceptable. 50k would be to much to me.

 

[john curl]

I agree PMA. However, we often use 50K for hi mid fi, because the stuff driving it needs the least load possible, being in the class B Xover zone due to Scott Wurcer's et al's tradeoffs.

 

[PMA]

I understand these concerns, some equipment profit from the higher load impedance, but IMO there are many good signal sources than can drive 10k without any degradation.

 

[john curl]

CTC preferred it, as well.

 

[Bonsai]

Bottom line is we should all expect modern source equipment (CD players, tuners, Phono stages) to easily drive 10k Ohms. The only exception is tube gear, where some products need a Hi-Z input.

 

[hags]

Bottom line is we should all expect modern source equipment (CD players, tuners, Phono stages) to easily drive 10k Ohms. The only exception is tube gear, where some products need a Hi-Z input.

I don't know why that sould be the bottom line.
I prefer a higher input impedance. Easier to drive and better frequency response. Anything from 25K to 100K is prefered. I think 10K is better suited for a "passive preamp".

I'm not positive but I think the input impedance of Q1,/Q19 and Q3/Q21 will support a higher input resistro than the 24K without any kind of problem. I wanted to ask to be sure though.

 

[hags]

No. The main reason why I prefer 10k is that lower resistance of the volume pot results in less change of frequency response as a function of pot angle, and also the input stage is driven from lower impedance, which I do prefer as well. But, 25k is still acceptable. 50k would be to much to me.

Pavel, you're talking about frequency change on the output side of the volume control, correct?

 

[AndrewT]

I don't know ......
I prefer a higher input impedance. Easier to drive and better frequency response. Anything from 25K to 100K is prefered. I think 10K is better suited for a "passive preamp".

............. I think the input impedance of Q1,/Q19 and Q3/Q21 will support a higher input resistro than the 24K without any kind of problem. .......

the capacitance following the pot determines the frequency response.
Hi pot values demand lower capacitance and thus put a physical limit on the way the components are assembled.

 

[hags]

the capacitance following the pot determines the frequency response.
Hi pot values demand lower capacitance and thus put a physical limit on the way the components are assembled.[/QUOTE

Sure, I understand that. With these particular devices where is the roll off going to begin with 25K versus a 10K pot?
Surely not audible.

 

[PMA]

No need to change the cap. 25k pot results in 6.25k output resistance worst case + 1K filter res + 220pF filter cap results in 99.7kHz (-3dB) worst case rolloff.

 

[AndrewT]

F-3dB @ <100kHz is getting a bit low.
I prefer F-3dB ~200kHz.
With 1k0 & 220pF, that would require
~zero capacitance from the cable and a 10kpot.

 

[hags]

PMA and AndrewT,
I'm guessing the capacitance of the active devices is so low as to not come into play?????

So the main concern in this application is the input filter?

 

[hags]

I am wrong to assume that I can do away with the input cap?

With say 100p from the cable and the 7.25 resistance I'd be at about 219kHz, no?

 

[AndrewT]

Hi,
the input cap lowers the HF impedance seen by the input devices.
Some designers say that this low input/source impedance is necessary to maintain amplifier stability.
Maybe PMA would care to comment on whether he follows that design philosophy and how his amp reacts to source impedance as against source resistance.

 

[hags]

Hi,
the input cap lowers the HF impedance seen by the input devices.
Some designers say that this low input/source impedance is necessary to maintain amplifier stability.
Maybe PMA would care to comment on whether he follows that design philosophy and how his amp reacts to source impedance as against source resistance.

Yes, and as I understand it also acts as part of an RF filter. I've seen this in alot of commercial products as well.

 

[PMA]

F-3dB @ <100kHz is getting a bit low.
I prefer F-3dB ~200kHz.
With 1k0 & 220pF, that would require
~zero capacitance from the cable and a 10kpot.

There is no cable behind the pot. Please first review before you advise.