LSK pre - BAF 2013

[sergiog]

Hi, Juma just finished assembling the post.67 chematic preamp V+/V- 33.5 vdc, Offset OK, added 220R resistor to R4-R5 now I kindly ask you how I can increase the gain (resistor value to be changed) since to hear the music at a decent level I have to turn the knob to about 1/2 volume
pot is Alps Black 50K.Best regards Sergio Italy

 

[juma]

Lowering the R6 will increase the gain and the distortion too.
Increasing the R9 will increase the gain also but it it will increase the output impedance (Zout) too.
Zout is determined by value of R9 and it is already very high (6k8 paralleled with Zin of the next stage).
I don't know what is your next stage (amplifier? , pot?) and what is its Zin, but it's possible that you don't lack gain - it might be that your next stage "eats" the gain (low Zin) so you actually might need a buffer rather than more gain in the preamp.
Since you are hiding data about your exact configuration it's hard to help you.

 

[sergiog]

Hi, the matching amplifier is the Pass F4

 

[william2001]

Nothing wrong with running a VC at halfway. Sound quality overall is probably better (resolution, detail) with a pot @50% as opposed to 15-20% where you are just throwing away excess signal in the form of heat through a resistive voltage divider.

 

[juma]

Hi, the matching amplifier is the Pass F4

OK, in that case the circuit needs a bit of rearrangement:

 

[Carl_Huff]

Looking back on the preamp schematic in post 66, how do we substitute for the BF862 part that has since disappeared?

 

[juma]

This one (2SK209 BL) will do nicely:
https://www.digikey.com/en/products...nductor-and-storage/2SK209-BL-TE85L-F/4305046

 

[Carl_Huff]

Thanks Juma,

I always learn something new from the projects you post, and I thank you for that.

 

[marconi118]

just for my understanding what are the advantage of feeding the signal to the bjts by the emitter kind of cascade instead of feeding it at the basis?

 

[juma]

Search the net for:

• common base/grounded base configuration
• transconductance amplifier

 

[Carl_Huff]

Juma,

I am studying your cicuit back in #66. I have two questions ....

Firstly why both 24 and 33 volts. Why not run the whole thing on 24 volts?

And secondly, have any changes (improvements??) been made since you posted that circuit?

Thanks a bunch for your contribution!

 

[sergiog]

Hi Juma in the schematic at post.705, I can remove the C1-C6 capacitors since there is the trimpot offset. Thanks again

 

[juma]

1. why both 24 and 33 volts.
2. Why not run the whole thing on 24 volts?
3. have any changes (improvements??) been made since you posted that circuit?

1. Having in mind max. V_out of the gain stage it would be wasteful (voltage/power-wise) to supply the buffer stage with more than +-24V.
2. Sure you can supply both stages with +-24V if you don't mind max. output voltage swing being lowered from roughly 15V_peak to about 10V_peak. In that case R4 and R5 should be changed to750R.
3. I think there were several, but I can't remember them all (it was >10years ago). The one in posts #485 and #492 comes to my mind (post #705 too) - for the rest you'd better browse the thread.
Cheers !

 

[juma]

Hi Juma in the schematic at post.705, I can remove the C1-C6 capacitors since there is the trimpot offset. Thanks again

Hi Sergio,
sure, you can leave those caps out if you are certain that no significant DC voltage can find its way to your speakers 👍