HT MC Phono Stage AD797

[jls001]

I started my build of Hypnotoad's MC Phonostage for low output MC cartridges. This design is based on the AD797 ultra low-noise and ultra-low distortion instrumentation opamp. The output stage is designed for the LME49990. As I am using 8-pin DIP socket adapter, there is of course scope for trying out other opamps.

Original build thread here: AD797 Phono Stage Build and Help Desk Thread - AudioKarma.org Home Audio Stereo Discussion Forums

The earlier Hypnotoad design was this one.

Some build pics:

Bare circuit board:

Circuit being populated:

Completely populated:

This is the Russian PETP 2.2 uF capacitor which Sachin had kindly sent to try out. I will first run the regular cap (WIMA) and then try the PETP later to hear the differences.

This build will see - for the first time for me - proper housing and proper power supply being used. Both my earlier phono stage builds - the CNC MM and Hynotoad MC - never got proper housing, and both were run off dual 9V batteries.

@sachu888: do I have to short the 4700 pF cap to the 47 Ohm resistor as shown in the first pic on this build page??? (see the red arrow marking at the bottom of the pic): AD797 Phono Stage Build and Help Desk Thread - AudioKarma.org Home Audio Stereo Discussion Forums

Also see pic on post #8 on same page.

Or is the shorting taken care of in this version of the board?

 

[sachu888]

@sachu888: do I have to short the 4700 pF cap to the 47 Ohm resistor as shown in the first pic on this build page??? (see the red arrow marking at the bottom of the pic): AD797 Phono Stage Build and Help Desk Thread - AudioKarma.org Home Audio Stereo Discussion Forums

Also see pic on post #8 on same page.

Or is the shorting taken care of in this version of the board?

Hi Joshua,
There was a missing trace in first batch of PCbs.This has been taken care of in the next batch.No problem with your board.

Regards,
Sachin

 

[aks07]

This is the Russian PETP 2.2 uF capacitor which Sachin had kindly sent to try out. I will first run the regular cap (WIMA) and then try the PETP later to hear the differences.

WIMA red box MKP's?

I have compared Russian K75 1uf pio caps with WIMA MKP's and Solens in high voltage DC blocking and parafeed role. The difference was stunning and immediately apparent. Bypassed with a 0.1u MKP.

No ideal on the K73 line as I have only seen K75 range in Indian junk shops.

The AD797 is a killer opamp.

All the best on your new build.

 

[sachu888]

WIMA red box MKP's?

I have compared Russian K75 1uf pio caps with WIMA MKP's and Solens in high voltage DC blocking and parafeed role. The difference was stunning and immediately apparent. Bypassed with a 0.1u MKP.

No ideal on the K73 line as I have only seen K75 range in Indian junk shops.

The AD797 is a killer opamp.

All the best on your new build.

K-73 is totally different from K-75 series.I bought it from a Ukraine seller.

Regards,
Sachin

 

[jls001]

I am using the Pass B1 cabinet built by magma for the group buy when the B1 GB kicked off. The cab is all aluminium, with a separate section for PS. It comes with an IEC socket with inbuilt fuse and on-off switch.

I need directions on how to wire up the mains.

The outside view of the socket is here:

The inside view is here:

My confusion is how to wire up the R-Core primaries to the switch.

One info seems to suggest this internal wiring in this IEC socket cum fuse cum switch:

So does it mean I connect live wire of R-Core Primary to A, R-Core Primary Neutral to B, and R-Core Primary earth wire to the terminal marked Earth?

Thanks in advance.

 

[sachu888]

Hi Joshua,
There are 6 wires (3 per side ) in transformer,Red,Green,Black for AC(for power switch) and Blue,White,Blue for DC (for power supply PCB).Below pics may be useful:

EDIT: don't forget to put 150 or 200 ma slow blow type of fuse in the socket.

Regards,
Sachin

 

[jls001]

Thanks Sachin for the prompt reply.

In your diagram, I feel that the live and neutral are reversed after the fuse. Strictly speaking, it doesn't make a difference - and will work perfectly (since supply is AC).

 

[suraj]

going slightly off topic,sorry for that.
i had a question in the wiring of the IEC socket.
Would adding a emi/rfi filter between the IEC socket and the transformer help in removing mains noise?
i mean,something like this
Elcom - RFI/EMI Filter, 15 Amp ( EP-15AP )

 

[sachu888]

going slightly off topic,sorry for that.
i had a question in the wiring of the IEC socket.
Would adding a emi/rfi filter between the IEC socket and the transformer help in removing mains noise?
i mean,something like this
Elcom - RFI/EMI Filter, 15 Amp ( EP-15AP )

No need for that our Power supply is having 6X10Mh Panasonic inductors to remove any unwanted noise.

Regards,
Sachin

 

[omishra]

No need for that our Power supply is having 6X10Mh Panasonic inductors to remove any unwanted noise.

Regards,
Sachin

Hi Sachin,
Still if he uses that it will keep ground of PS at low noise as it will be cut off before entering transformer. So using it will not harm.
Regards

 

[jls001]

Minor progress report:

1) adjusted the phono stage's input impedance for a Denon DL 103 cartridge. I chose 103 Ohms:lol: Recommended loading is 100 Ohms.

2) adjusted the gain of the phono stage for a 0.3 mV output cartridge, which is about 1000 Ohms (on the 0 to 2000 Ohms Bourns pot). Final value is 1003 Ohms on both channels. With patience, it is possible to adjust the resistances of the input and gain to exact desired values.

I could not rig up the OPS yet as I don't have the correct screw to secure the R-Core and PCB to the cab floor. Hopefully I should be able to get it today. I tried 9V batteries but it didn't work as both the pairs were drained as they have been in storage for quite some time now.

Also, I am integrating the Pass B1 buffer in the same cabinet. The board is fully populated. I have also received the input selector circuit so I think it makes sense to do that too.

 

[Tanoj]

1) adjusted the phono stage's input impedance for a Denon DL 103 cartridge. I chose 103 Ohms Recommended loading is 100 Ohms.

2) adjusted the gain of the phono stage for a 0.3 mV output cartridge, which is about 1000 Ohms (on the 0 to 2000 Ohms Bourns pot). Final value is 1003 Ohms on both channels. With patience, it is possible to adjust the resistances of the input and gain to exact desired values.

I got the MC phono stage from Sachin, and my cart was Denon DL 160

I wish to know where and how to check and adjust the input imp and gain etc

I got Multimeter ( Digital )

Thank you

Tanoj

 

[jls001]

I got the MC phono stage from Sachin, and my cart was Denon DL 160

I wish to know where and how to check and adjust the input imp and gain etc

I got Multimeter ( Digital )

Thank you

Tanoj

Please see post number 7 here.

The DL160 being a high output MC (1.6 mV), is designed to work with MM cartridge phono preamps. A phono preamp with a gain of 40-44 dB and loading of 47 kOhms must be more than sufficient for it.

 

[sachu888]

Please see post number 7 here.

The DL160 being a high output MC (1.6 mV), is designed to work with MM cartridge phono preamps. A phono preamp with a gain of 40-44 dB and loading of 47 kOhms must be more than sufficient for it.

He is using 1st version of HTMCPS,which is perfect for HOMC.

Regards,
Sachin

 

[sachu888]

Hi Joshua,
any update?

Regards,
Sachin

 

[jls001]

Nopes

Been too tied up over the past weekend. Will get back to it this week. Thanks for the troubleshooting tips.

 

[jls001]

Progress report & request for troubleshooting help:

I have managed to rig up the circuit many days ago, and tried powering it with dual 9V rechargeable batteries, but it didn't work. The input and output RCAs are temporary female RCA sockets wired with think coax cables.

After several misadventures including the discovery that both input cables are shorted, and trying incompatible opamps, I think I have got the circuitry right, but still no sound.

The following are already checked:

1) input and output cables changed from coax to twisted pair taken from a Belden CAT6 patch chord.

2) all resistor values manually checked and found normal.

3) polarity of all electrolytics checked and verified OK.

Still no sound.

Some pics below.

This is how it looks after changing the in-out cables to twisted pair:

A zoom of the above photo showing a closeup of the circuit board:

Another closeup showing the opamps (LME49990 on the left, AD 797 on the right):

A couple of things more to try:
1) try a fresh pair of 9V batteries (non-rechargeable)
2) try powering it with OPS power supply

 

[sachu888]

Hi Joshua,
also check Lme49990,on DIP adapter.They should be seated firmly there.It happened once with me.
EDIT: Did you measure voltage on +v,G and -ve on PCB? Kindly remove toggle and try to power directly to board.

Regards,
Sachin

 

[jls001]

Hi Joshua,
also check Lme49990,on DIP adapter.They should be seated firmly there.It happened once with me.
EDIT: Did you measure voltage on +v,G and -ve on PCB? Kindly remove toggle and try to power directly to board.

Regards,
Sachin

Seating - tried tightening all opamps multiple times.

I didn't measure voltages. Will try tonight. And I will also measure them at the V+ and V- points of opamp sockets without the opamps.

I will eliminate the DPDT too. I do have a spare DPTD in case it is the culprit.

 

[linuxguru]

Not directly relevant to this circuit, but for a ~1mV swing MC cartridge a low-noise JFET pre-input stage like 2x 2sk170 or 2x BF862 in parallel will give a much lower voltage-noise floor than the hot-running bipolar AD797. At such low swings, distortion becomes irrelevant and the limiting factor is the input-stage voltage noise of the opamp, which is significantly higher than a large-geometry low-noise JFET.

You can check out a JFET 2-stage pre- with RIAA equalization by Salas at diyaudio here (there are other designs with opamps for the RIAA also):

Simplistic NJFET RIAA - diyAudio

The second stage can be a high-performance low-noise opamp like LT1028, OPA627, AD797 or LME49990. Even quieter and darker will be a fully-discrete folded-Kaneda Class-A opamp with an ultra-low noise bipolar input stage like my LF05 single.