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I finally got around to completing a schematic (less power supply) for a full-featured control amp. I've been working on various aspects of this for some time. It's unity gain (actually about -0.1 dB) with low output Z, switchable tone controls and built-in RCA-type phono preamp. This is a draft schematic only. Some component values will need to be tweaked, particularly the EQ in the phono where I've substituted the Russian 6N2P for 12AX7s.
I'm planning to construct this in a fairly deep cabinet. All tubes will be mounted horizontally on brackets that span the chassis left-to-right. That will allow easy revision or even replacement of any individual section.
As always, comments welcome.
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Buy Chinese. Bury freedom.
Follow Ups:
I'd love to hear your impressions of the DC servo vs. cap coupling on the preamp.
CCS's in your phono preamp will give you more gain and less THD. Using a tube with a little less gain but lower Rp and higher transconductance may be worth considering as well.
RIAA EQ is closer to "ideal" if 24 K, instead of 22 K, is used in the network.
If you plan on recording phono, the low level mag. section needs to be buffered.
Eli D.
OK, I'll start with 24k and tweak from there if needed. Yes, I've thought about including "Phono Out" jacks. That would require another high quality buffer though, and it doesn't seem reasonable to duplicate that effort when the control amp already has one. I'll probably just add a second pair of output jacks. That will provide a line level phono signal at the output when the gain control is at maximum. The power amp can then be attenuated externally (or with its own volume control, if present) to reduce speaker level. This isn't a function I'm likely to use often, so I think that's the most reasonable way to handle it.
Thanks for your input!
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Buy Chinese. Bury freedom.
The tweaked RCA phono setup I'm associated with will drive a recording device. ;> )
The tweaks address 2 of the 3 weaknesses in the RCA original. Grid leak biasing the 2nd gain block lightly loads the EQ network and improves bass extension. The MOSFET source follower deals with the original's very poor drive capability. See RCA's remarks. The 3rd weakness is the high C Miller of the 'X7 triode, which can adversely interact with some carts. and SUTs. Dealing with that requires a tube and EQ network change that I didn't want to get involved with. The 6GK5 is (IMO) an obvious candidate for the alternative 1st gain block role.
FWIW, those who have built the tweaked setup have been pleased. Check the archives.
Eli D.
Thanks, I'll keep those points in mind when I'm building that section. I'm also interested in Allen Wright's approach (FVP5), and I'd like to hear a zero-bias 5755 phono stage. That will come later, after I have a version of the RCA that I can live with. Just ordered a few new aluminum structural components, so this will all hopefully start to take shape in the not-too-distant future. :)
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Buy Chinese. Bury freedom.
Early on in this project, I was considering a timed relay at the output to prevent thumping the power amp. Then I moved to the DC coupled output design, and now I'm wondering if a delay is necessary. In this design, applying power before the output stage is warm creates a negative output voltage. Into an open circuit, -30V will be present (although no more than 13 mA can flow due to the resistance of the choke). Driving a tube amplifier with this, I don't foresee a problem. Even without a coupling cap at the input, the worst that can happen is to put the input stage into cutoff. I'm wondering though, are there SS amps that might be damaged or adversely affected by this? It's been so long since I've worked with SS amps, I just don't remember all the various iterations.
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Buy Chinese. Bury freedom.
Yes, throwing DC out of your preamp isn't a good plan, especially if you plan to use a solid state amp. Here's a solution:
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