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Build Phono Preamp

There are two schematic for phono preamp, which one can I should chose?
Thanks.
 

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Don't build the first one as is. The decoupling resistor for the first stage (to the right of where it says "266V") is only 1k ohms, which is far too small unless you change the 100uF capacitor to something like 470uF.

The second one has the 12AX7 tubes biased almost into grid current, with only -0.44V grid to cathode of the first stage 12AX7.

Rather than try to fix those problems, why not build something tried and true? How about Eli Duttman's classic RCA-style 12AX7 RIAA preamp?

Tweaked RCA Phono Preamp Schematic - Eli Duttman - Tube DIY Asylum
 
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Code:
1k with 100uf  has a  Cut-off frequency of 1.5 hz.Not low enough?How low do you want?


Change 1k to 100k and 100u to 22u, will have better ripple rejection, drop plate from 266v to 210v still performs the same.


If you disconnect the 12ax7 in the 2nd stage, the output of this stage should stay the same as when it's connected, this mean the riaa performance is not dependent on loading (say input Z of 12ax7). The first sch does just that, maybe not Eli's sch.
 
A cutoff at 1.5Hz means its only filtering rail ripple (~100Hz) by 36dB, for an amp handling signals in the microvolt to millivolt range that's not necessarily enough ripple rejection - the cutoff frequency isn't the focus, the attenuation of ripple is.

Whats the total filtering with the circuit posted as is?,with the 27k and 10uf filter preceding the 1k 100uf ,Thanks.
 
Oh wow! I'm criticizing my own circuit! Well, I've learned something since I posted that a couple of years ago...

After building a similar 12AX7 phono preamp with a passively filtered power supply (no regulation), I found that the first stage 12AX7 is -extremely- sensitive to power supply impedance down around 1Hz (yes, one cycle per second). I was using something like 4.7k ohms and 33uF, which theoretically should be low enough, but there were weird rumbles happening down low from subsonic oscillations ("motorboating"). I wrote about it on this forum, and one kind person pointed out that the decoupling (dropping) resistor feeding the B+ to the first stage in a phono preamp needs to be relatively large in order to avoid these oscillations. He recommended at least 15% of the value of the first stage triode's plate load resistor. 15% of 150k is 22.5k, so there you go. I found that I needed 43k in the particular circuit I ended up building, since a 33uF electrolytic cap was already in the PCB (and it fit snugly too). Upping the decoupling resistor to 43k cured the motorboating.

If you like that first circuit, here's an updated version that fixes those problems. I modeled it with a +320VDC B+ supply with 100mV of ripple riding on it. The output waveform shows no trace of ripple, so the power supply decoupling shown should work to filter out ripple and have low enough impedance not to oscillate at subsonic frequencies. For what it's worth, it's attached (the schematic on the left).

I'm also including a schematic of the RIAA preamp I'm listening to now. It's nothing special but I think it would be a good novice build, because it uses a passive power supply and tubes you can buy at Guitar Center. It was designed around 12AX7s, but I found that the roughly 200pF input capacitance is too much for my Audio Technica AT-VM95E cartridge, which has a recommended loading of not more than 250pF total (200pF 12AX7 Cin + 110pF tonearm cable + 30pF internal tonearm wiring = 340pF). So I put in a 12AT7 and I'll live with the reduced gain. The highs definitely sound more relaxed now, so I'm happy enough to listen to it. I am working on something better, though...

I forgot to mention that my current RIAA preamp was built using a pair of Merlinb's Compact Phono PCBs. Merlin's PCBs are good -- they're well thought out, they're well made, and they're not expensive. If you can scrounge up some 12AY7 or 6072, that would make a nice phono stage if you don't need gain of > 40dB. 12AY7 was often used in microphone preamps, so was designed for low noise. It also features much lower input C than 12AX7, so will play nicer with moving magnet cartridges. Or use a 12AT7 like I ended up doing. It distorts more, and doesn't have quite as low input C, but it can be made to work pretty well. The MOSFET source follower output frees you from worries about loading by cable runs or low impedance amplifier inputs.

The Valve Wizard
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If you disconnect the 12ax7 in the 2nd stage, the output of this stage should stay the same as when it's connected, this mean the riaa performance is not dependent on loading (say input Z of 12ax7). The first sch does just that, maybe not Eli's sch.

Good point Koonw!

Yes, a larger Rseries in the RIAA EQ network does present a lighter load to the input stage 12AX7. Perhaps that's why I prefer the sound of the "12AX7" preamp I built with 12AT7 instead?

Perhaps it's necessary to make the EQ Rseries 10X the resistance of the preceding stages internal plate resistance (rp).

12AX7 rp = about 70k ohms
12AT7 rp = about 25k ohms

If Rseries = 200k ohms, then that is < 3X the rp of the 12AX7/ECC83. The 12AX7 is probably loading down into the EQ network.

If Rseries = 200k ohms, then that is 8X the rp of the 12AT7/ECC81. That is probably not loading down the 12AT7.

Oh well. Back to the drawing board...
 
Continuing the thought from my last post...

Looking at my preamp in LTspice:

With a 12AX7 as the first stage, measuring the 1kHz output from that first stage 12AX7's plate:
- In circuit (the RIAA EQ loading the 12AX7) the output level at the first stage is 350mV peak.
- With the RIAA EQ circuit bypassed (not loading the 12AX7) the output level from that first stage is 408mV peak.

That's a drop of 14% in output voltage at 1kHz.

With a 12AT7 as the first stage (drawing only 1mA Ip), measuring the 1kHz output from that first stage 12AT7's plate:
- In circuit (the RIAA EQ loading the 12AT7) the output level at the first stage is 294mV peak.
- With the RIAA EQ circuit bypassed (not loading the 12AT7) the output level from that first stage is 271mV peak.

That's a drop of only 8% in output voltage at 1kHz.

With a 12AT7 biased to 2mA Ip as the first stage, measuring the 1kHz output from that first stage 12AT7's plate:
- In circuit (the RIAA EQ loading the 12AT7) the output level at the first stage is 299mV peak.
- With the RIAA EQ circuit bypassed (not loading the 12AT7) the output level from that first stage is 315mV peak.

That's a drop of only 5% in output voltage at 1kHz.

I wonder what is an acceptably heavy loading on the input stage? I'm sure as light a load as you can get will be the best way to go, but when can you have perfection? I guess the answer is to put a follower on the output of the input stage, so that driving the passive RIAA network becomes relatively easy -- and don't go crazy with a low impedance RIAA network because that 'sounds better'. Everything is a compromise between this and that...
 
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Attached is the sim of CF and normal loading of 2nd stage riaa. I believe if the level drop is less than riaa deviation level (in this case about 0.25/0.4 db), all is fine?.
 

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Just love your "copy````left````" :D:D

All wrongs reserved :D

Realistically, I mean it to mean that you can make, reproduce even commercially this design, but you must include credit ("circuit by kodabmx" or the like) and you must freely provide the schematic of your design complete with any modifications.

If there was a microcontroller involved, the code would also be required to be open source and freely available.

I might be mistaken though, but I don't think I can release a schematic or PCB under the GPL license since it's for software.
 
Hi

take consideration also
Junior phono

and I suggest always to consider the project with the test lab done as a certification

Absolutely!

- This circuit puts a cathode follower on the output of the input stage, which solves the problem of the input stage loading down into the passive RIAA EQ network.

- This circuit also has a cathode follower on the output, making it less likely to load down or slew limit into long cable runs or solid-state power amplifiers or powered speakers with low input impedance.

Is there a PCB available for purchase for this product?
Is there a power supply kit to go with it?
--
 
What happens if you put the CF *before* the RIAA EQ, but leave the EQ output going to a common cathode 12AX7?

Perhaps try a 12AU7-CF DC coupled from first stage 12AX7's plate.

See my schematic in post 7.

I found it was less noise to put majority of the gain AFTER the RIAA filter. 6N1P is quite a quiet tube and the filter can be more "linear" because the voltage swings are lower.

You could build that circuit in post 7 with 12AU7 instead of 6N1P and 12AX7 instead of 6N2P - you will just have less gain and higher distortion - but no parts value changes are needed.
 
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