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Old 22nd April 2008, 02:09 AM   #1
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Default Copland CTA-402 Phono Section Comments

I got at a very good price an almost brand new Copland CTA-402 Integrated Amplifier. Here the phono section for your comments.
It is using great parts quality: Beyschlag film resistors, Phillips MKP capacitors (now Vishay, same as used by Lamm). For the RIAA active feedback network, Phillips foil film and polystyrene caps. The input cap is MKC (I guest polycarbonate) and the output coupling cap is an Evox MKP.

It is using a few nice tricks, one single triode per channel, final section bias using a diode, input triode reference point lift by about the same 0.6V using a resistor, active RIAA network feedback applied directly to the input triode.
I read that this is probably the simpler phono preamp you can build, just one tube per channel. Its mean disadvantage is a rather high output impedance. But in this case, it is not really a problem because it is built into an integrated amplifier with a high input impedance active preamp. The output goes directly to the balance 100K potentiometer, then the volume pot and the preamp input.

I'll gladly receive your comments on this preamp and how it can be improve.
I think the best thing to improve is the output coupling cap, the Evox MKP. I think to bypass it with a 0.1uF Jensen Copper foil and a MK1837.

I'll do some measurements on the RIAA correction curve accuracy tomorrow.
I checked it rapidly using an inverse RIAA network and an input square waveform. It looks perfect at 1Khz.

Let me know what you think.

Thanks in advance.
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Old 22nd April 2008, 12:21 PM   #2
SY is offline SY  United States
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Gut and rebuild. Designer parts are nice, but the basic stage is very problematic, having the same issues as the late and unlamented Dynaco PAS.

1. Input capacitance is high. Many MMs will not be happy, and any sort of MC stepup will suffer.

2. Loading of the 12AX7s is quite low (like 1.5x rp for the first stage), so the open loop distortion will not be very good.

3. Open loop gain is very, very marginal. If you tweak the RIAA feedback for conformance, the low frequency distortion will be quite high.

4. The RIAA network makes the loading issue on the second stage a real problem. Think about what the effective plate load is at high frequencies, where the RIAA caps are low impedance. At low frequencies, the load is better, but still far too low (50k!).

5. The 0.7V diode biasing in the second hole will almost guarantee poor overload performance. Especially at high frequencies, where the second stage will be gasping for air anyway because of the cruel load.

RIAA stages are very difficult to design properly, which is why there are so few good commercial designs. This is not one of them.
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Old 22nd April 2008, 02:13 PM   #3
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Quote:
Originally posted by SY
Gut and rebuild. Designer parts are nice, but the basic stage is very problematic, having the same issues as the late and unlamented Dynaco PAS.

1. Input capacitance is high. Many MMs will not be happy, and any sort of MC stepup will suffer.

...
Hi SY,
ups, are you able to tell me a value for the input capacitance?
Thanks.

Kind regards,
Darius
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Old 22nd April 2008, 10:52 PM   #4
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Thanks SY for the comments. Very informative indeed . I guest, I'm still better with an EAR834P dedicated phono stage
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Old 22nd April 2008, 11:19 PM   #5
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One other choice- you could probably build up something like Allen Wright's circuit, a FET-tube cascode, inside the Copland.
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Old 23rd April 2008, 12:03 AM   #6
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Originally posted by SY
One other choice- you could probably build up something like Allen Wright's circuit, a FET-tube cascode, inside the Copland.

SY,

AW's JFET "downstairs"/triode "upstairs" seems oriented towards LOMC service. If the phono section is to be mated with a MM cart., could things be better using a 6922 section "downstairs" and a MOSFET, say a ZVN0545A, "upstairs"? With both cathodes of the 6922 close to ground, issues of heater to cathode potential limits are "off the table". Obtaining sufficient gain from the cascode appears to be unproblematic, given 6922 gm of 12.5 mA./V.
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Old 23rd April 2008, 12:07 AM   #7
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I would need some kind of schematic to do so, since my tube knowledge is rather limited. Any to suggest?
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Old 23rd April 2008, 12:17 AM   #8
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Algar, poke around Allen's site (www.vacuumstate.com). Buy his preamp book- it's idiosyncratic, but an interesting and useful read.

Eli, Allen gives parts values for MC and MM. FWIW, I used a circuit very similar to his (but with an NTE458 on the bottom) for about 20-25 years with various MMs. Worked fine in that service. The low input capacitance came in handy when it was time to install an MC stepup when I changed over to LOMC.
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Old 23rd April 2008, 12:27 AM   #9
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From an Allen white paper I got this schematic. It can probably be mounted directly using the existing PCB. For the record I tested the accuracy of the RIAA response of this preamp. It is really good and match perfectly the curve, here some results:

Since there is no preamp output, measured at the output into 8ohms load at 1V output level, 1Khz reference.
Audio generator terminated into 50ohms load. I agree this is not an actual cartridge.

Freq(Hz)Theor(dB) Actual(dB)
50 +16.93 +16.6
100 +13.09 +13.0
200 +8.23 +8.20
281 +5.88 +5.90
398 +3.81 +3.8
500 +2.63 +2.6
1000 0.00 Ref
1413 -1.17 -1.1
1778 -2.07 -2.0
2818 -4.36 -4.2
3981 -6.55 -6.3
5012 -8.18 -8.1
6310 -9.92 -9.8
7943 -11.72 -11.4
10000 -13.55 -13.4
12590 -15.37 -15.3
15850 -17.17 -17.2
19950 -18.89 -19.4
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Old 23rd April 2008, 12:52 AM   #10
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Seems a good choice here. I built something similar based on a design by Steve Bench. The Allen Wright one only needs one dual triode, so would be easy to fit in your chassis. Here's the full article for the schema above: http://vacuumstate.com/various/SP-15_Article.pdf

As shown above, you'll only need the front end. And your RIAA values will be very close if you load it with 50kOhm. You will need a very quiet power supply, as the cascode has no PSRR to speak of. Or, of you are up for it, you can try some noise canceling techniques, as described here: http://www.tubecad.com/march99/.

Sheldon
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