buildafriend
Well-known member
mitsos said:
I can measure and confirm the DCR for you about the 2092. If you care for it still.. since I have one.
RCA Ba2A Pre amp qestions & answers
- Thread starter gary o
- Start date
Help Support GroupDIY Audio Forum:
I can measure and confirm the DCR for you about the 2092. If you care for it still.. since I have one.
Yes. I think that's their best one and I would recommend it over the B version.
Real BC2 is about 50db of gain. Both Cinemag inputs have less gain than original. 10C will have slightly more gain than PCA. Original OT is 7:1 so compare that to Cinemag to see where gain loss stands. You'll likely end up around 40db of gain.
Assuming you have right spec power transformer I would look at what happens to bottom end with the stock cathode resistor of V2. You should be drawing around 3-4ma on V2. You can adjust cathode resistor for less current and slightly better bottom but it may not net you huge improvement. I'd forget what is said about the Cinemag OT "being good up to 8ma" or whatever that value is. 3-4ma is plenty for driving high-Z loads and the bottom response gets slightly worse as current goes up.
buildafriend
Well-known member
Thanks lassoharp. I added some voltages to my post near the bottom of page 17. Maybe you guys will spot something weird about them. Should I add any other test measurements?
mitsos
Well-known member
- Joined
- May 4, 2007
- Messages
- 2,886
I can measure and confirm the DCR for you about the 2092. If you care for it still.. since I have one.
[/quote]
hey man, that'd be cool. Still haven't gotten around to winding an output for my "BA2" so this info would be nice to have. thanks!
buildafriend
Well-known member
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buildafriend
Well-known member
Finaly I got around to make a version of this preamp. And it works!! And it sounds very good...big open, lots of detail.
I used a EF37a tube with a 600/100k JS input transformer. Output is a 6SJ7 to a LL 1680/5ma transformer.
Old Siemens 0.5mf PIO cap inbetween. Used a guitar amp chassiss from Tube Town...a bit large but with the power supply on the other end this is dead quiet. This will be my goto pre for vocals from now on.... Thanks to everybody on this thread for inspiration and help.
J
I used a EF37a tube with a 600/100k JS input transformer. Output is a 6SJ7 to a LL 1680/5ma transformer.
Old Siemens 0.5mf PIO cap inbetween. Used a guitar amp chassiss from Tube Town...a bit large but with the power supply on the other end this is dead quiet. This will be my goto pre for vocals from now on.... Thanks to everybody on this thread for inspiration and help.
J
Hi,
got my BA-2 back home to try different input transformers, i did many listening tests between them and also run frequency response tests.
Inputs are CMMI-10C:
http://cinemag.biz/mic_input/PDF/CMMI-10C.pdf
and CMMI-10B:
http://cinemag.biz/mic_input/PDF/CMMI-10B.pdf
Output is kind of CM-27101 with gapped iron core and several other differences like max 10mA DC.
Both inputs with this OT have -3dB points at ~35Hz and 28kHz. CMMI 10C has a bit better response between this two points, but this differences are very small and might be caused by my error, although i'm pretty sure i got it right this time.
Both inputs were also measured out of circuit, properly loaded and with 20dB pad (this part is the same as when they were in circuit).
Results are pretty much the same as in datasheets, but i found out 10B's response falls much faster than 10C's under 20Hz. One of more interesting things i don't really understand is that 10C has higher -3dB point at 82kHz, 10B has it much higher at 115kHz.
Higher frequency response can clearly be heard on 10B, highs are more pronounced compared to 10C. Whole picture is kind of shifted to highs, although it still has bass and all, just different. I like the way 10B sounds with this particular combination of trafos, circuit and applications, but would like to understand better why this is so. Looking at older inputs i don't often see response to so high frequency, they are a bit similar to 10C, sound is also less toppy, except for those tiny Beyer.
I also exchange some PMs about this matter and was told modern transformer designers seem to design for much more highs than needed i guess. Measurements and listening tests confirm this to a point, of course i should meassure and listen to many more models to really confirm this. Anyone has any thoughts about it, do you want me to measure some more as i have a day or two more before preamp goes back to the studio? To me it looks a bit like OTs where bigger cores have bigger bass.
Like i said, 10B sound very nice for my needs and i also tried another OT which was a little too clean for what we are doing. I love BA-2, open and lots of detail is very good description and i just started understanding what it can do with different iron, bias, RL, output loading, etc.
got my BA-2 back home to try different input transformers, i did many listening tests between them and also run frequency response tests.
Inputs are CMMI-10C:
http://cinemag.biz/mic_input/PDF/CMMI-10C.pdf
and CMMI-10B:
http://cinemag.biz/mic_input/PDF/CMMI-10B.pdf
Output is kind of CM-27101 with gapped iron core and several other differences like max 10mA DC.
Both inputs with this OT have -3dB points at ~35Hz and 28kHz. CMMI 10C has a bit better response between this two points, but this differences are very small and might be caused by my error, although i'm pretty sure i got it right this time.
Both inputs were also measured out of circuit, properly loaded and with 20dB pad (this part is the same as when they were in circuit).
Results are pretty much the same as in datasheets, but i found out 10B's response falls much faster than 10C's under 20Hz. One of more interesting things i don't really understand is that 10C has higher -3dB point at 82kHz, 10B has it much higher at 115kHz.
Higher frequency response can clearly be heard on 10B, highs are more pronounced compared to 10C. Whole picture is kind of shifted to highs, although it still has bass and all, just different. I like the way 10B sounds with this particular combination of trafos, circuit and applications, but would like to understand better why this is so. Looking at older inputs i don't often see response to so high frequency, they are a bit similar to 10C, sound is also less toppy, except for those tiny Beyer.
I also exchange some PMs about this matter and was told modern transformer designers seem to design for much more highs than needed i guess. Measurements and listening tests confirm this to a point, of course i should meassure and listen to many more models to really confirm this. Anyone has any thoughts about it, do you want me to measure some more as i have a day or two more before preamp goes back to the studio? To me it looks a bit like OTs where bigger cores have bigger bass.
Like i said, 10B sound very nice for my needs and i also tried another OT which was a little too clean for what we are doing. I love BA-2, open and lots of detail is very good description and i just started understanding what it can do with different iron, bias, RL, output loading, etc.
Attachments
Your measurements on the bottom don't sound out of line with the RCA catalog spec.
For mic pad in and 620 ohm load (best bottom), my original RCA is -3 about 12Hz, -2/16, -1/27. Worst loading perspective is -3/18, -2/25, -1/43. That's better than specified by RCA. I don't have measurements of others to know if mine is better than average or not.
Top extension: modern designers are also chasing linear phase which requires linear frequency. The original has a treble resonance boost around 15K with certain load conditions.
For mic pad in and 620 ohm load (best bottom), my original RCA is -3 about 12Hz, -2/16, -1/27. Worst loading perspective is -3/18, -2/25, -1/43. That's better than specified by RCA. I don't have measurements of others to know if mine is better than average or not.
Top extension: modern designers are also chasing linear phase which requires linear frequency. The original has a treble resonance boost around 15K with certain load conditions.
emrr said:
Thanks, very helpful as are your measurements i have in toolbar for a long time:
http://groupdiy.com/index.php?topic=41032.msg513738#msg513738
I didn't focus so much on OT loading this time, when i did at least two good models with proper driving stages didn't complain about loading (600r-1k). One of better UA converters had ~2k input impedance which looks strange in days of bridging, but this is another thing, it made me think at a time they maybe wanted to get "more sound" this way...
Also checked some Sowter, Jensen and all Cinemag inputs i could find. They seem to confirm your remark about top end extension, many "better" models have phase vs frequency plots and not all of them are on sites. I think we have enough choice this days, limit seem to be experiences with all this new and old iron.
I promised report of new nickel pin EH EF86 after some time in circuit. Now they have year and a half of quite frequent use and a lot of testing (turning on and off, heaters are reg., HT cap. multipliers, micalex sockets). A few samples work well with the same current, no microphonics or changes of any kind, only slight expected change in current in first days or so. One runs as pentode, i think this tells good news anyway.
Why triode strap a pentode as opposed to using triodes?
Was it for supply reasons? I see that the Ba1a uses 1620s as pentodes (and I assume it was in production first), so maybe this is why? Is there any sonic advantage?
Was it for supply reasons? I see that the Ba1a uses 1620s as pentodes (and I assume it was in production first), so maybe this is why? Is there any sonic advantage?
Radio Designers Handbook addresses this I believe. The 1930's RCA use the 77 predecessor in triode 90% of the time, they could have easily gone with a triode.
It was as well for supply reasons, easier to stock one type.
BA-1/2/3/4/6 all came out the same time. BA-1/2/4 all exist in previous incarnations with different model #s in the 80 series.
No reason not to build a BA-2 with a single 6SN7, really.
It was as well for supply reasons, easier to stock one type.
BA-1/2/3/4/6 all came out the same time. BA-1/2/4 all exist in previous incarnations with different model #s in the 80 series.
No reason not to build a BA-2 with a single 6SN7, really.
vari-mu
Well-known member
emrr said:
I use in my preamps, similar RCA BA-2,
low voltage sub-miniature military tubes 6N28B-V
(Mini 6SN7, I think) :
Have made 30-40 products different configurations and constructions, mainly amplifier + optical compressor.
Here a two-channel preamp into 1RU :
My basic scheme (similar RCA BA-2) :
Examples sounding guitar (2009) :
http://moemesto.ru/vari-mu/file/9283267/1%20Pre%20El%20Gtr%20.wav
Vladimir P.
Vari-mu
Wow. . . A beautiful build. Have you tried it on vocals yet? I tried to listen to the link you put up of guitar sounds, but it wouldn't play.
Wow. . . A beautiful build. Have you tried it on vocals yet? I tried to listen to the link you put up of guitar sounds, but it wouldn't play.
burdoto
Well-known member
I built a Ba6a circuit with some not-ideal transformers and the I have pretty large bass roll off. I think the roll off starts around 1k and is eventually -10db at 60Hz. I've bypassed the input transformer and I've bypassed the first tube stage but no change in the rolloff. It could be the output tranny which is a single ended edcor job( GXSE5-15K which I've since read is not great for this purpose). Any ideas? Output tranny? I also built the op6 circuit similar to the preservation sound guy, which has different output tranny setup, and same bass roll off. What have I done wrong? Any help is appreciated.
That Edcor looks like it's intended for SE 6BQ5 or 6V6 and just right for flea watt guitar amp duties but the sec is given as a 600r only (no speaker taps). Interesting.
You can possibly get better bottom response from running the output tube at much higher current to lower the source impedance. Running it at 10 -12 ma may knock it down to 8-9K which is better than the 12-14K it normally runs at.
You'd probably be looking at a rebuild or separate project but that Edcor OT would be just right for running 6BQ5 or 6V6 triode strapped. Dave P designed a SE 6BQ5 mic pre and I think the thread and schematic are still up in the archives.
You can possibly get better bottom response from running the output tube at much higher current to lower the source impedance. Running it at 10 -12 ma may knock it down to 8-9K which is better than the 12-14K it normally runs at.
You'd probably be looking at a rebuild or separate project but that Edcor OT would be just right for running 6BQ5 or 6V6 triode strapped. Dave P designed a SE 6BQ5 mic pre and I think the thread and schematic are still up in the archives.
