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In Reply to: RE: Buffer - emitter resistor value? posted by 13th Duke of Wymbourne on December 24, 2024 at 13:59:45
I've just changed the emitter resistors in my phono stage from 1k ... to 4k7; listening to the phono stage with the arm lifted off the LP ... this has considerably reduced the noise I hear. So that is a biiig win!
Thinking about it, I presume (with a +24v DC rail) there must've been 24ma flowing through the BC547C, when the emitter res was 1k. Only having 1/5th of that current flowing through it now (with 4k7) ... makes it quieter.
Follow Ups:
I would not expect the follower noise to be significant. In cascaded stages the source noise gets amplified down the chain and each stage adds its own noise but the noise floor is raised with each stage of amplification hence succeeding stage noise requirements are relaxed. The first stage is the most important and in communication systems the first stage is always a low noise amplifier (LNA).
For simplicity, let's ignore the amplifier noise floor and just think about the noise of the signal source. If it is a LOMC the source resistance is ~ 10ohms and the noise of that resisitance will get amplified. Assuming you have at least 40dB of voltage gain (100x), the noise power after amplification will be equivalent to a resistor of 10*100*100 = 100k. I cannot imagine your follower producing anywhere near that level of noise (datasheet says it has 2dB noise figure referenced to 2kohms that means it is bit noisier than a 2k resistor). So, a mystery that you could decipher using spice noise analysis or just enjoy another LP
you may well think it " a mystery that I could decipher using spice noise analysis or just enjoy another LP ".
All I know is that after I changed the 1k emitter resistor to 4k7 ... the noise that I was hearing from the spkrs with the stylus lifted off the LP was low enough to make listening to an LP, a pleasure. Whereas the noise that I heard from the spkrs when the 1k emitter res was in place ... made it impossible to enjoy listening!
And re. removing the 'M' from my description, as I obviously know very little EE theory ... Audioasylum require me to have that 'M' designation as, even though I am merely a retired bloke with a hobby, the fact that I make and sell a few components means I have to be called a 'manufacturer'!
Andy, sorry if I've offended. 'M' was a small elephant in my room in that someone can make stuff without really knowing how it works. But I get you're doing this as a hobby and it is great that you're selling something. Selling something makes you a more successful audio designer than me! I met the LKV guy a couple of times at shows in SoCal and he was a hobbyist during his lawyering career and then turned audio into a business when he retired. The second time I met him he told me he was at least breaking even, which is great if you can perform your hobby without going broke. I haven't seen anything of LKV in the audio press for a long time so I hope all is well.
That said, I am happy to provide insights as, IMHO, those are the most important things and they can take a long time to acquire (well, they have for me but I've worked with a lot of really smart people who pick them up quickly). I made the joke about 'M' because the answer I gave you about HOMC was mostly gleaned from a web search and wasn't anything you couldn't have found out yourself. FYI, there are probably better sites to pose technical questions such as DIYaudio and PFM. I read PFM and there are technical posters there. I don't go to DIYaudio as it is full of people nerdier than me and I'd have to fully engage or stay away.
And I wasn't having a dig about the spice noise analysis. As I said, I would not expect the output follower to set the noise floor so when you change the bias current you should not hear a difference. Understanding why things don't work as expected as where one can gain insight and I could be wrong (I make no claim to infallibility). Spice can rank the noise contributors in the circuit so you'd see where most of noise comes from. I don't think low noise design is that necessary in audio even for phono preamps because once the needle hits the record even the quietest vinyl will create more noise than from the electronics.
Talking of phono playback, I remember your skeletal LP12 from The Wam. Last year I upgraded to a Supatrac Blackbird tonearm on my LP12 and it has been the most wonderful improvement.
And I greatly appreciate your input.
The phono stage I build is certainly not a 'business'. I sell a few each year and my sell-price is based on parts cost + A$10 an hour for my labour (so it's by no means a 'profit maker')!
As such, it outclasses in SQ, phono stages up to 5 or 6x the price. I actually have a customer in SoCal, if you're interested in hearing it. :-))
I certainly agree with you that " once the needle hits the record even the quietest vinyl will create more noise than from the electronics ". However, achieving a lower noise floor is my current "windmill tilt" ... I want to achieve as low a noise-floor as my backup phono stage has (a Hagerman 'Euphonia').
Fascinating that you found a reference to my 'SkeletaLinn'. It gets gradually improved, as time advances - and someone has just built one in the UK; he reckons it already outclasses his LP12 ... and he's still got more tweaking to go. :-))
The Supatrac Blackbird seems to be a great tonearm; if I didn't already have my 12" 'Univector' (UP) - which was the reason I designed my 'SkeletaLinn' - I would probably get one, too. Unfortunately, the designer/builder of the Univector died about 5 years ago ... so they are no longer available.
Here's a pic of the current state of the SkeletaLinn' - on its recently installed air isolation platform:
The 12" Univector is at the front; a Magnepan 'Unitrac' (also a UP) is at the back right. Current chassis is made of a Panzerholtz-like ply ... which you may know Linn recently used for their new plinth.
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