Values for R13 and gain.
Firstly thanks to 6L6 and others for another excellent build guide and of course NP himself for the circuits. Great inspiration to build.
Just finished biasing my BA3 pre and realised I've made a bit of a mistake on R13 and overall gain.
My existing pre (Primare 32) has a gain of 16dB which is way too much for the rest of my system. So I was looking at building the BA3 pre as a replacement. (I'm currently using a B1 Buffer after the pre but would like to reduce the box & cable count). This will drive my F5.
24V rails (Glassware +/- 24V PS) and all other values as per stock except 220 Ohms for R13. Which I calculated (R10/R13) to give 10 x gain.
It sounds really good but is a bit low in terms of max volume so I'm thinking to increase R13 to 330 as per stock value but wondering what this relates to in dB terms. (Apologies but I'm not sure how to work this out).
But seeing earlier reply giving stock gain of 7 should I perhaps look at increasing R13 further to 440 or higher??
Patience & support appreciated as ever.
Thanks Giles
Firstly thanks to 6L6 and others for another excellent build guide and of course NP himself for the circuits. Great inspiration to build.
Just finished biasing my BA3 pre and realised I've made a bit of a mistake on R13 and overall gain.
My existing pre (Primare 32) has a gain of 16dB which is way too much for the rest of my system. So I was looking at building the BA3 pre as a replacement. (I'm currently using a B1 Buffer after the pre but would like to reduce the box & cable count). This will drive my F5.
24V rails (Glassware +/- 24V PS) and all other values as per stock except 220 Ohms for R13. Which I calculated (R10/R13) to give 10 x gain.
It sounds really good but is a bit low in terms of max volume so I'm thinking to increase R13 to 330 as per stock value but wondering what this relates to in dB terms. (Apologies but I'm not sure how to work this out).
But seeing earlier reply giving stock gain of 7 should I perhaps look at increasing R13 further to 440 or higher??
Patience & support appreciated as ever.
Thanks Giles
Values for R13 and gain.
When vol turned up full it is an OK listening level but on the quiet side. You feel you want to turn it up! No where near blast your ............. (ears) off.
That is with my DAC input (3V RMS output). Its too quiet with my FM radio input, 1V or 750 mV I think.
I'm happy to do a little minor corrective surgery but would like to be a bit more certain about what I'm aiming for.
Thanks for the quick response.
When vol turned up full it is an OK listening level but on the quiet side. You feel you want to turn it up! No where near blast your ............. (ears) off.
That is with my DAC input (3V RMS output). Its too quiet with my FM radio input, 1V or 750 mV I think.
I'm happy to do a little minor corrective surgery but would like to be a bit more certain about what I'm aiming for.
Thanks for the quick response.
I also built stock BA-3 Front end (R13 = 330R) with non Toshiba parts and get a gain of 10 times or 20 db.
In Papas article he gets 30 db:
"The voltage appearing at the Gate of Q3 is amplified by something less than the ratio of R10 divided by R8, and with the same happening at Q4 and considering the transconductance of the Mosfets, comes out at about 15. Both of them added make a system voltage gain of about 30X, or 30 dB."
Maybe you can measure the voltage at the input and the output of the gainstage to see, which gain you actually have now. Maybe there is something going on with the attenuator.
In Papas article he gets 30 db:
"The voltage appearing at the Gate of Q3 is amplified by something less than the ratio of R10 divided by R8, and with the same happening at Q4 and considering the transconductance of the Mosfets, comes out at about 15. Both of them added make a system voltage gain of about 30X, or 30 dB."
Maybe you can measure the voltage at the input and the output of the gainstage to see, which gain you actually have now. Maybe there is something going on with the attenuator.
Last edited:
Values for R13 and gain.
I'd elevated the board to accommodate larger 10uF output caps. So R13 is on the underside of the board due to its size and snagging with the heatsinks (removed in pics, as are the channel selector & vol pot connecting rods).
It was a bit of work to splice in (series) additional R values to bring R13 up to 330 but finally plucked up courage and it went well.
Then disaster!! I fritzed R10 & 11 on the left hand board when I was setting up to rebias. My croc clip leads were dangling over the side and shorted!! There's a lesson here
Very fortunate really that is was just the two Resistors that got fried. As they were raised i was able to replace them from above (not so pretty but functional) and rebias.
Full vol level is much better - a good listening level. Not too loud though.
Then I remembered My F5 is not at full bias/load as it has 50V rails and even with fan cooling in a modded enclosure I've been a bit conservative to keep fan noise as low as and a hand safe temp on the sinks.
So I can go back to rebias the F5 if needed as well as looking at a further increase in R13.
Its still only biased to 900mV and burning in but sounds really (really, really) good. Very open and airy with good detail.
Thanks for all the info in the posts and support.
Cheers
Giles
I'd elevated the board to accommodate larger 10uF output caps. So R13 is on the underside of the board due to its size and snagging with the heatsinks (removed in pics, as are the channel selector & vol pot connecting rods).
It was a bit of work to splice in (series) additional R values to bring R13 up to 330 but finally plucked up courage and it went well.
Then disaster!! I fritzed R10 & 11 on the left hand board when I was setting up to rebias. My croc clip leads were dangling over the side and shorted!! There's a lesson here
Very fortunate really that is was just the two Resistors that got fried. As they were raised i was able to replace them from above (not so pretty but functional) and rebias.
Full vol level is much better - a good listening level. Not too loud though.
Then I remembered My F5 is not at full bias/load as it has 50V rails and even with fan cooling in a modded enclosure I've been a bit conservative to keep fan noise as low as and a hand safe temp on the sinks.
So I can go back to rebias the F5 if needed as well as looking at a further increase in R13.
Its still only biased to 900mV and burning in but sounds really (really, really) good. Very open and airy with good detail.
Thanks for all the info in the posts and support.
Cheers
Giles
Attachments
if you're chasing higher voltage level, then why you need preamp, in first place?
considering that I don't have info about architecture of analog stage of said DAC**, I can't asy are there benefits of using BAL out, putting voltage level aside
so , just use it's SE output
**if you can find further info about that, we can think more
considering that I don't have info about architecture of analog stage of said DAC**, I can't asy are there benefits of using BAL out, putting voltage level aside
so , just use it's SE output
**if you can find further info about that, we can think more
Separate from method used etc...
Zout RCA - 100ohm
Zout Balanced - 200 ohm
Source -
TOPPING D70s MQA DAC (Digital-to-Analog Converter) – Apos Audio
Zout RCA - 100ohm
Zout Balanced - 200 ohm
Source -
TOPPING D70s MQA DAC (Digital-to-Analog Converter) – Apos Audio
Hi Folks,
Today I picked up a long abandoned project of mine, a BA3 preamp to try to get it working. My problem is with only one channel, the other works and sounds fine. The bad channel has been double checked to make sure all the correct resistors are in the correct spots, that Q1 to Q4 are correct and has been reflowed. It biases up the same as the good channel, have the bias at 1v with the offset ranging between - 5 mv and +5 mv. One channel sings the other is silent. The psu holds a steady 24 vdc with both channels hooked up. Before I start pulling jfets and mosfets out of the board does anyone know if I can test them is situ. If that is not possible I found what appears to be a matched set of jfets from the diyaudio store in my parts bin and I have some spare mosfets as well. Thanks
Today I picked up a long abandoned project of mine, a BA3 preamp to try to get it working. My problem is with only one channel, the other works and sounds fine. The bad channel has been double checked to make sure all the correct resistors are in the correct spots, that Q1 to Q4 are correct and has been reflowed. It biases up the same as the good channel, have the bias at 1v with the offset ranging between - 5 mv and +5 mv. One channel sings the other is silent. The psu holds a steady 24 vdc with both channels hooked up. Before I start pulling jfets and mosfets out of the board does anyone know if I can test them is situ. If that is not possible I found what appears to be a matched set of jfets from the diyaudio store in my parts bin and I have some spare mosfets as well. Thanks
Attachments
