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Old 17th June 2013, 02:32 PM   #151
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Default Increasing gain

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Originally Posted by Russ White View Post
Hi PD,

Great questions from you as I would expect.

You have a good grasp of the circuit. Nothing I can find to correct you on.

I will post the values I am currently using in my test rig below. Keep in mind. This cct is still in development. So now is the perfect time to get good input from folks like you. I don't have PCBs made yet, so plenty of time for change.

Right now I am using another PCB that I have modified to fit the bill (MESH) to do my prototype testing.

For now here are some numbers.

IC1 is OPA1632, IC2 is OPA2228(Though I am open to suggestions). AD8620 is one I will also try. There is also a new dual JFET input opamp from TI (I forget the part at the moment) that looks promising.

All resistors are .1% where good matching is required.

R1/R3 22K R2 is 1K. This yields a differential gain of 33db for the instrumentation amp.

This RIAA filter/diff input block is as follows:
R5/R6 are 493R
R9/R10 are 1.5K
R4 is 105R
C1 is 1uf 1%
C2 is 3uf 1%

The feedback H network is as follows:
R11-R15 are all 10K for MM. (system gain is 39.1 db at 1khz)
R13 is 470 for for low output MC. (system gain is 59.9db at 1khz)
R13 is 2.21K for high output MC. (system gain is 47.9db at 1khz)

As I said before. These are all preliminary values. But they seem to be working very well for me so far.

Cheers!
Russ
Hi Russ,

I have built the kit and I would like to know:

1.- How to increase gain (it is just changing the value of R13?)
2.- In my kit there were two different sets of the big dayton caps, one set was with 5% tolerance and the other set was 1% tolerance, I installed the former (5%) but I do not know if I have don correctly...

Best Regards
Jorge Castellano
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Old 17th June 2013, 02:54 PM   #152
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Quote:
Originally Posted by jcastellano View Post
2.- In my kit there were two different sets of the big dayton caps, one set was with 5% tolerance and the other set was 1% tolerance, I installed the former (5%) but I do not know if I have don correctly...
When I first bought kit parts, only the 5% was available. Since then, the 1% have become available so I got some and have been adding them to the kits. Either will work fine, but the 1% is preferable, as they are more likely to be close to the intended value.
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Old 18th June 2013, 02:38 PM   #153
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Default Balanced to SE

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Originally Posted by jcastellano View Post
Hi Russ,

I have built the kit and I would like to know:

1.- How to increase gain (it is just changing the value of R13?)
2.- In my kit there were two different sets of the big dayton caps, one set was with 5% tolerance and the other set was 1% tolerance, I installed the former (5%) but I do not know if I have don correctly...

Best Regards
Jorge Castellano
Hi Russ and Brian,

I think I have made a mistake. I asked before how to increase gain, because I had the impression that the sound was a little "thin" without punch so I thought it was a matter of gain....but I have come across in the TP forum that, as the kit provide balanced output, it is not desirable to use only one phase (out+ and gnd), it is better a bal to se conversion. Please confirm...

Brian, thank you for your fast response. I appreciate!!

Regards,
Jorge
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Old 20th June 2013, 12:08 PM   #154
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Yes, definitely best to convert the balanced signal to SE if you need SE.
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Old 27th June 2013, 01:22 PM   #155
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Originally Posted by Russ White View Post
Hi Folks,

I can't count how many times I have been asked when I would design a phono stage. Well I really don't know why it took me so long. But here it is.

The Retro is a unique design which uses an instrumentation amplifier front end with a 47K input impedance so as not to overload MM magnetic cartridges. It is equally well suited to MC cartridges because its gain can be easily increased to over 60db with little penalty.

The RIAA filter is reasonably accurate, at least in simulation. It should also be in practice since 1% caps and .1% resistors are used.

The fully symmetrical nature of the circuit means that distortion is low and common mode input noise is rejected very well. You can still use normal RCA inputs with one end of the cartridge output grounded. I have been using it with the cartridge floating. I use XLR with pin 1 connected to the retro GND and pins 2 and 3 connected only to the cartridge outputs. One per channel.

I used a "H" network to raise the gain of the fully differential stage without using too high value feedback resistors or overly loading the instrumentation amp. This lowers the noise and distortion a bit.

The output is balanced, but you are free to only take one side if you like and make it single ended.

Joshua Tree works wonderfully at the output to make a complete preamp.

It sounds awesome.


Cheers!
Russ
About 20 years ago I wrote an article for Glass Audio about a design for an RIAA preamp I had developed on my own. I reasoned that the phono cartridge was in reality a mechanical microphone and that a balanced connection between the cartridge and the preamp was a good idea, since balanced lines were developed for exactly this circumstance (low signal levels in a high noise environment) way back in the 1930's. I used an input transformer to convert the balanced signal to SE and then used a SE RIAA preamp with passive EQ.

Some cartridges have a strap between the shell and one of the minus pins. That strap needs to be removed for this system to work well. Also, it is advisable to re-wire the turntable with tiny shielded twisted pair cable (I used "tape head wire") and install XLR connectors on the back of the turntable base. They have mini-XLR connectors now that would be very good to use.

I also found the sound to be very natural with low noise, so this is a wise and good choice by Twisted Pear Audio.
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Old 28th June 2013, 10:46 PM   #156
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Thanks Dirkwright!

I had some family and friends over yesterday and we were listening to lots of vinyl using Retro and Joshua Tree directly into dual mono Sympatico power amps. A good time was had by all!

A common comment is how low the noise floor is. It really does present a nice clean stage. This is a project I have enjoyed quite a lot.

I appreciate your kind remarks.

Cheers!
Russ
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Old 28th July 2013, 04:11 PM   #157
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Russ,

Do you have any boards available for the low-Z MC-only version?

Greg in Mississippi
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Old 29th July 2013, 04:01 PM   #158
benpe is offline benpe  Poland
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I am using low MC cartridge (Lyra Dorian), I have put R13 1k with switch, making parallel connection with additional resistor 100R (good for 0.6mV of my cartridge output).
For 0.3mV I would put something like 47R there. Cartridge load - I have added resistors directly at the sockets, in my case 100R.
I hope Russ can comment it, am I doing something against the rules here ?

Thanks !
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Old 31st July 2013, 01:59 AM   #159
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Benpe, that should work just fine. I would be interested to know how that sounds.

Greg, I don't have any of the low z boards right now, but I would be happy to get some made if people want them.
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Old 31st July 2013, 07:42 AM   #160
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Dear Russ,
I am also interested in a Retro Complete Kit

Have a great day,
GM
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