AD797 Phono Stage Build and Help Desk Thread

Thanks Wyn!

If you're talking about the .asc file from the Leach web page, that's for the Common-Emitter circuit, not the Common-Base that I built (he didn't provide .asc files for the 2 Common-Base circuits). So your reference to "change R1 to 75 and R4 to 10k" may not be appropriate to my build.

I don't have the means to take real measurements, so I used the "Mark-I ear" method. With everything turned on and warmed up, needle lifted, turned the volume to maximum. Had to get my ears to within 6-8 inches of the speakers to just begin to hear hiss (and a little hum). An old analog RS SPL meter wouldn't even register 60dB w/mic 1" from either tweeter or woofer panels.

All-in-all, I'd say thats very quiet!
bypassing the degeneration R to
Will still do the AD797 though, and see how they stack up against one another.
Ah! It looks like I should build circuits for all of his flavors.
The common base design is far more dependent on the exact characteristic of the transistors than the common emitter, and the miller capacitance of the input transistors will appear across the input of the common emitter design, but other than that the audio performance should be very similar.
The load R should also be scaled about the same to provide the increased gain.
I'll find some time over the next few days and do a full simulation of it.
 
Thanks Wyn!

If you're talking about the .asc file from the Leach web page, that's for the Common-Emitter circuit, not the Common-Base that I built (he didn't provide .asc files for the 2 Common-Base circuits). So your reference to "change R1 to 75 and R4 to 10k" may not be appropriate to my build.

I don't have the means to take real measurements, so I used the "Mark-I ear" method. With everything turned on and warmed up, needle lifted, turned the volume to maximum. Had to get my ears to within 6-8 inches of the speakers to just begin to hear hiss (and a little hum). An old analog RS SPL meter wouldn't even register 60dB w/mic 1" from either tweeter or woofer panels.

All-in-all, I'd say thats very quiet!

Will still do the AD797 though, and see how they stack up against one another.
I simulated the common base version of the Leach amp. As expected, it's very sensitive to the exact operating point of the input transistors, so I changed R2 to 449k- which is what should be required to give a nominal device current of 125ua- and adjusted R5 to 6.2k to give a gain of 34dB with the Ortofon cartridge/interconnect and the loading of the next preamp stage. The A weighted noise is 5.3uv, which is very, very slightly better (less than 0.5dB) than the Leach CE design and the worst case AD797 design.
By the way, the CE design needs a larger load R (the 10k) as it uses two 30k bias Rs that are effectively in parallel with each other and with the load R.
 
... so I used the "Mark-I ear" method. With everything turned on and warmed up, needle lifted, turned the volume to maximum. Had to get my ears to within 6-8 inches of the speakers to just begin to hear hiss (and a little hum). An old analog RS SPL meter wouldn't even register 60dB w/mic 1" from either tweeter or woofer panels.
All-in-all, I'd say thats very quiet!
Yes, I'd agree that seems quiet. Just one caveat - it's the signal-to-noise ratio that really matters, not the absolute noise. That means if your system has a lot of gain, ie the volume control only needs to be at about the 9 o'clock position for the music to be too loud, there's bound to be audible hiss/hum when you max out the volume control.
On the other hand, if your system has a lot less gain and the volume control needs to be flat out just to get a reasonable volume, it's not surprising that hiss/hum is inaudible.
 
I simulated the common base version of the Leach amp. As expected, it's very sensitive to the exact operating point of the input transistors, so I changed R2 to 449k- which is what should be required to give a nominal device current of 125ua- and adjusted R5 to 6.2k to give a gain of 34dB with the Ortofon cartridge/interconnect and the loading of the next preamp stage. The A weighted noise is 5.3uv, which is very, very slightly better (less than 0.5dB) than the Leach CE design and the worst case AD797 design.
By the way, the CE design needs a larger load R (the 10k) as it uses two 30k bias Rs that are effectively in parallel with each other and with the load R.
Thanks Wyn!

I started a Leach specific thread, so we can let HypnoToads AD797 RIAA stage get back on-topic. I'll move over there.

http://audiokarma.org/forums/index....-moving-coil-pre-preamp.838004/#post-11975083

Sean
 
Yes, I'd agree that seems quiet. Just one caveat - it's the signal-to-noise ratio that really matters, not the absolute noise. That means if your system has a lot of gain, ie the volume control only needs to be at about the 9 o'clock position for the music to be too loud, there's bound to be audible hiss/hum when you max out the volume control.
On the other hand, if your system has a lot less gain and the volume control needs to be flat out just to get a reasonable volume, it's not surprising that hiss/hum is inaudible.
Well, the Apt preamp was overly sensitive to begin with, 9-10 o'clock was typical listening, 11 o'clock was as loud as I could stand.

Fortunately, the users manual had instructions for reducing sensitivity by increasing the output resistor values, which I did.

So now 1 o'clock is ideal for my MM cart, but it needs 3+ o'clock for the Leach/MC20 to get the same volume. In either case, I hear nothing in-between tracks.
 
Getting ready :)
3r2xz1S.jpg


Regards
Sachin
 
Hi guys, Well this morning I got Jan Didden's Silent Switcher up & running & have to say S*t this unit is super, super quite.
Yes this little unit has made my AD797 phono amp furnominally quite & it has also, to my delight made a significant improvement to it's performance & dynamics, with more fine detain being heard. In fact bass is so impact-full that I may have to turn down my sub's. This silent switcher was destined for another project, but not now, it is staying where it is. In fact it is so small, I can build it into the current case, by mounting it on the lid.

Cheers Derek
 
Hi all,
Has anybody here tried to make a balanced aka differential configuration of this phono stage with two boards? I'm not sure the boards are prepared for this or if some tweaking would be necessary. I would like to avoid the output capacitor (a major offender) and a true balanced or differential setup, with no drift (ie zero DC offset), would allow that.
Thanks for any feedback on this.
 
Hi all,
Has anybody here tried to make a balanced aka differential configuration of this phono stage with two boards? I'm not sure the boards are prepared for this or if some tweaking would be necessary. I would like to avoid the output capacitor (a major offender) and a true balanced or differential setup, with no drift (ie zero DC offset), would allow that.
Thanks for any feedback on this.
Yes, I designed a circuit which avoids the DC coupling cap which is based on the AD797MCpre board. It uses an additional opamp which feeds the low passed output from the second gain stage back to reduce the output offset. It uses the same 2.2u cap as the AC coupled design except that the cap is now in shunt rather than series. The circuit is not "differential" and I confess to not understanding what you mean in your description of a balanced design. The offsets that the current board exhibits are largely due to non-deterministic effects, particularly if you use some of the resistor changes that I previously recommended, paired with the very large DC gain of the circuit, and two "balanced" input channels would not have matched offsets that could be cancelled.
 
This is the circuit. It also has some modifications to the way the RIAA is performed. The best, theoretical, way to implement an RIAA is to do all of the equalization in the first gain stage- I won't go into why as it's a lengthy explanation- but that's not so good when the first amp is an AD797. As an alternative the best approach is to put the 75us pole first- and in some ways it could be argued that it's actually better- and that's what I did. The gain is 72dB and it's compliant to RIAA within +/-0.1dB 20-20kHz.
The DC offset is designed to be less than 10mv worst case...

DCoff.PNG

I built this and It seems to work well, but I haven't checked it sufficiently to see if I "just got lucky".
The DC offset values are the max values based on my cartridge/phono cable combo which has a 16 ohm DC series resistance.
 
Hi guys, Well this morning I got Jan Didden's Silent Switcher up & running & have to say S*t this unit is super, super quite.
Yes this little unit has made my AD797 phono amp furnominally quite & it has also, to my delight made a significant improvement to it's performance & dynamics, with more fine detain being heard. In fact bass is so impact-full that I may have to turn down my sub's. This silent switcher was destined for another project, but not now, it is staying where it is. In fact it is so small, I can build it into the current case, by mounting it on the lid.

Cheers Derek
Have you tried measuring the frequency response of the preamps with and without the switcher? It's really hard for me to believe that the bass is quantitatively different as the opamp PSRR at low frequencies is enormous- about 1uV/V at DC and I see no sag or ripple on the supplies after the linear regular in my system.
By the way, I don't use a transformer- I use a Meanwell PD-2515 switcher directly from the AC mains followed by the linear regulator. I see no difference in the output noise floor/frequency response if I use this combo versus a pair of 12V batteries, and no audible difference that I can determine.
 
Hi wyn, No sorry, I cannot measure the frequency response. I just rely on my ears, so yes it is subjective & yes it may be the feel good new shoes syndrome. However I can confirm it staying.

Cheers Derek
 
spartan1969

As nearly all, if not all preamp stages (check the schematic of the line amp first) have dc blocking caps at the line inputs you could try bypassing the output capacitor as it's only there to stop any dc offset from entering the line amp. I have tried this and didn't notice any difference, but my ears are old. :D On my Yamaha CR-1020 they have 10uf electrolytics. :eek:

I did find a basic schematic for the AD797 which is pretty much up to date and shows how simple the circuit is, which I did on purpose, if anyone is interested:


Schem-1.png
 
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spartan1969

As nearly all, if not all preamp stages (check the schematic of the line amp first) have dc blocking caps at the line inputs you could try bypassing the output capacitor as it's only there to stop any dc offset from entering the line amp. I have tried this and didn't notice any difference, but my ears are old. :D On my Yamaha CR-1020 they have 10uf electrolytics. :eek:

I did find a basic schematic for the AD797 which is pretty much up to date and shows how simple the circuit is, which I did on purpose, if anyone is interested:


Schem-1.png
Thank you!
 
spartan1969

As nearly all, if not all preamp stages (check the schematic of the line amp first) have dc blocking caps at the line inputs you could try bypassing the output capacitor as it's only there to stop any dc offset from entering the line amp. I have tried this and didn't notice any difference, but my ears are old. :D On my Yamaha CR-1020 they have 10uf electrolytics. :eek:

I did find a basic schematic for the AD797 which is pretty much up to date and shows how simple the circuit is, which I did on purpose, if anyone is interested:


Schem-1.png
As for the o/p capacitor, if the amplifier you're connecting the phono preamp has an input cap, then removing the o/p cap from the phono stage will make no difference other than an increase of the total capacitance. Unless there's a huge difference in the quality of the caps in question. My initial question presupposes a pure DC connection btw the parts... Then the difference is substantial.
 
As for the o/p capacitor, if the amplifier you're connecting the phono preamp has an input cap, then removing the o/p cap from the phono stage will make no difference other than an increase of the total capacitance. Unless there's a huge difference in the quality of the caps in question. My initial question presupposes a pure DC connection btw the parts... Then the difference is substantial.

You could put a relatively large cap in series with the resistor to ground at the inverting input of the output amp.
This will reduce the output offset to more or less just that of the input- which if the impedances are correct will be less than 1mv . The cap has to be non-polar, but it has a signal across it which is both very low frequency and very small, so distortion is extremely small and inconsequential and ONLY at frequencies close to the -3dB point and their harmonics/intermod products. If you really wanted to you could shunt it with smaller value "better quality" caps.
If the resistor is 390 ohms, then a 47u cap will give a -3dB point at 6.5Hz, a 22u at about 15Hz, etc.
The feedback amp I previously added only has the advantage of requiring a smaller cap and providing a "true" DC coupled design. Personally, I don't think it matters, but some do.
 
....Do not intend to steal the thread but since my question relates to this project that im trying to complete...What would be the best way to implement Mono-Stereo Switch ? :bowdown:
 
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