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Old 18th January 2012, 06:49 PM   #1
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Default 6SL7 / 6SN7 Phono

I have built a phono amp and it doesm not sound as I suspected and as promised the high frequencies sound compressed and there is no air around the instruments. Are there comments or remarks about the scematic that I use?
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Old 18th January 2012, 07:03 PM   #2
SY is offline SY  United States
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Are the RIAA values correct? How did you compute them? I calculated the last pole quickly and got 90us instead of 75, as an example.
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Old 18th January 2012, 07:23 PM   #3
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I have zero experience with designing or calculating the riaa part of the amp
so if someone can help me out with that part i would be very grateful
The phono amp is not my design , i found it on the internet

Last edited by brommermartin; 18th January 2012 at 07:27 PM.
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Old 18th January 2012, 07:38 PM   #4
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Rather than try to fix a fundamentally flawed design, I'd find a good one to start with. What sort of cartridge are you using?

It should be said that designing and building a good RIAA stage is the toughest problem in audio electronics.
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Old 18th January 2012, 08:02 PM   #5
Jebem is offline Jebem  Portugal
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Also, one possible reason is the last stage using that cathode follower... it can sound as you described when it is overloaded by an low input impedance from the next audio equipment.
What audio amp or preamp are you using?
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Old 18th January 2012, 08:22 PM   #6
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The cartridge i use is the Ortofon blue,
Specifications

•Output voltage at 1000 Hz, 5cm/sec: 5.5 mV
•Channel balance at 1 kHz: 1.5 dB
•Channel separation at 1 kHz: 25 dB
•Channel separation at 15 kHz: 15 dB
•Frequency range at - 3dB: 20-25.000 Hz
•Frequency response: 20-20.000 Hz + 2 / - 1 dB
•Tracking ability at 315Hz at recommended tracking force: 80 µm
•Compliance, dynamic, lateral: 20 µm/mN
•Stylus type: Nude Elliptical
•Stylus tip radius: r/R 8/18 µm
•Tracking force range: 1,6-2,0g (16-20 mN)
•Tracking force, recommended: 1,8 g (18 mN)
•Tracking angle: 20°
•Internal impedance, DC resistance: 1,3 kOhm
•Internal inductance: 700 mH
•Recommended load resistance: 47 kOhm
•Recommended load capacitance: 150-300 pF
•Cartridge weight: 7.2 g

The next audio part is my EF6 triode mode parafeed preamp with a input impedance of 47 kohm
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Old 18th January 2012, 08:35 PM   #7
SY is offline SY  United States
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OK, that narrows things down. You'll want an MM stage with low input capacitance. That means a cascode or (at the least) a first stage with a lower mu triode than 6SL7 or 12AX7. And you'll want something for which there are circuit boards. Allen Wright's preamps are a good choice- the FVP would do well for you. Ditto John Broskie's Aikido phono. There's probably more.

Your cathode follower will work fine into a 47k load- your problem is RIAA conformance and input capacitance.
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Old 18th January 2012, 08:41 PM   #8
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Quote:
Originally Posted by brommermartin View Post
I have built a phono amp and it doesm not sound as I suspected and as promised the high frequencies sound compressed and there is no air around the instruments. Are there comments or remarks about the scematic that I use?
I'm not surprised: that design is hideous. The first 6SL7 is loaded way too heavily. The design nominal plate resistance for this type is 44K, and with a 100K plate resistor, and the RIAA network, the effective plate load is right around 62K. Triodes like very light loading in order to operate in a linear manner. That would be bad enough, but the cathode degeneration drives the effective plate resistance even higher, making the loading that much worse.

The second 6SL7 stage still has a barely adequate 100K plate load, and that would be the case without cathode degeneration. Adding in the extra plate resistance makes that 100K far too small for this type. I've used 6SL7s before, and I like a much lighter plate load. That's simply the nature of the beast: the lighter the plate load, the better the linearity to the practical maximum of CCS plate loading. The 6SL7 is one of the better audio triodes, but it doesn't work magic, and it can not compensate for a fundamentally bad design.

This was designed by someone who really didn't know what he was doing. There is a lot of that going around these days, and tubes aren't transistors. Unlike transistors (or vacuum tube pents) what you hang on the cathode affects the plate characteristics (and vise versa). Here, the designer overlooked that detail.

I'd give that design Click the image to open in full size. Click the image to open in full size. Click the image to open in full size. Click the image to open in full size. out of five.
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Old 19th January 2012, 12:58 AM   #9
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One gadget that you'll find essential for making a phono stage is an inverse RIAA network. This allows you to test your results within the limitations of ordinary test equipment, even a computer soundcard, if you're lucky. There are a good variety of likely candidates in passive circuits fed at roughly line level in, phono out. If you make two (for stereo) you can do a listening test, with the inverse network and phono equalizer vs. without, maybe in a tape loop.

It sure can't sound right until you get the inverse network plus equalizer *flat*, so that's the place to start looking, as SY has said.

All good fortune,
Chris
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Old 19th January 2012, 02:04 AM   #10
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The groovewatt uses 12AX7 SRPP input driving the RIAA comp section feeding another 12AX7 SRPP stage followed by the 12AT7 cathode follower. I'm using this design with a home brewed PCB for my next project. I've got a set of 12AD7's along with several 12AX7's flavors to try out for tube rolling.
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