12AY7/6SN7 preamp PSU check - diyAudio
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Old 19th October 2005, 11:46 AM   #1
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Default 12Ay7/6SN7 preamp PSU check

Hi all-
I was wondering if someone could give this PSU a look over to see if there is anything obviously wrong with it? It looks ok to me, but I'm still learning.
I saw this preamp from Joel on another thread, and it looked simple enought to make as I have most of the parts.
My difference is I'm using a PT from an old Hallicrafters S-40A (don't worry, it's my spare parts donor and in no way pristine).
The spec's on the transformer are 330-0-330 secondary @67mA, so it has a little more voltage than I need. This is the reason that I had to change the original power supply posted with this schematic.
Thanks in advance.
Glenn

Here's the PSU:

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Here's the circuit:
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Old 19th October 2005, 02:54 PM   #2
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I'm too lazy to calculate the ripple. What did the sim predict?
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Old 19th October 2005, 03:39 PM   #3
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I didn't know that the PSU Designer did ripple calculations
I was more concerned that the original PS had the 20 & 100uF capacitors reversed. I thought that 100uF was too large for a 5Y3GT rectifier.
Glenn
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Old 19th October 2005, 03:47 PM   #4
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Well, you have 1.5K between the rectifier and the input cap, so you're unlikely to have an issue- the ripple current is limited by that resistor.

To get ripple, look at the voltage across C2/I1 after the initial settling in (a 5 second reporting delay will do fine). The sim gives min, max, and the difference. That difference is the ripple.
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Old 19th October 2005, 03:58 PM   #5
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Hi Sy-
Min is -52.962mV, max is 273.65 Delta is 273.7
Does that mean the ripple is 273.7V???? Something doesn't seem right.
If I zoom in on the graph of the voltage at I1, I only see ~0.1V ripple.
Glenn
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Old 19th October 2005, 04:02 PM   #6
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No, it means that you didn't put in a reporting delay time. Use 5 seconds, then you'll get sensible ripple numbers.
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Old 19th October 2005, 04:08 PM   #7
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OOOPS! Sorry, you did say that
Yes, that's better, now I get 84.947mV
Smooth enough for a preamp?
Glenn

BTW: V. Sattui & Domaine Chandon are my 2 most favorite places to vist when I'm there
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Old 19th October 2005, 04:28 PM   #8
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Interesting, when I swap the 2 capacitors (100uF then 22uF) I get 46mV ripple.
This is the way it was in Joel's original schematic. I guess it was right after all.
Glenn
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Old 19th October 2005, 06:08 PM   #9
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Well, it's all a matter of the S/N ratio you want to achieve. What you need to do is calculate the expected noise from the power supply rejection. In the case of a cathode follower, the rejection is approximately mu, so with a 6SN7 (mu = 20), you'd expect the ripple to contribute about 2.3mV of noise.

I don't know the 12AY7 specs offhand (I'm not at my own computer at the moment so I can't grab the data sheet), but what you want to do is get the plate resistance and mu at the tube's operating point. With those numbers in hand, you can see that there's a voltage divider formed by the load resistance (essentially, the 100K plate resistor) and the tube's source impedance, rp + (u+1)Rk, where Rk is the cathode resistor. Using a tube whose specs I do know from memory as an example (12AX7), the plate resistance is about 60K, the mu is about 100, so the source resistance would be 60K + (101) 1.5K = 210K. In that case, the rejection would be something like 210/(100 + 210), i.e., not very good. So you'd get a contribution of another 30mV or so from that stage. Referenced to a 3Vpeak output, that's a S/N of about 40dB.

Hopefully, the 12AY7 will calculate out better....
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Old 19th October 2005, 08:19 PM   #10
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Thank you very much for that information.
I'll have to look at the spec's and do the math.
I thought this was the case, how much you could live with at the speaker end, after all, it is an amplifier
Thank you.
Glenn
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