Hi,
I have an SAC Gama Phono pre for repair here (massiv bad noise). The input Opamp a THAT 1512 is used. The 1512 has a gin of -6dB. What is the advantage of teh 1512 over the 1510 (0 dB gain). The next stage must add the lost -6dB. For me, that makes no sense.
regards
Dieter
I have an SAC Gama Phono pre for repair here (massiv bad noise). The input Opamp a THAT 1512 is used. The 1512 has a gin of -6dB. What is the advantage of teh 1512 over the 1510 (0 dB gain). The next stage must add the lost -6dB. For me, that makes no sense.
regards
Dieter
You overlooked the gain setting resistor Rg.
With this single external resistor, gain can be set gain for both versions up to 60 dB.
For newer applications the 5512 is recommended because of lower noise.
When doing that, you will have to adjust Rg, roughly by a factor 2.
Hans,
Rg is open, so -6dB. Can you point me to the THAT5512 page ? There is no 5512 on the THATCORP homepage
Regards
Dieter
Rg is open, so -6dB. Can you point me to the THAT5512 page ? There is no 5512 on the THATCORP homepage
Regards
Dieter
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5512 was a typo, I meant the 1512.
I can't imagine a phono preamp based on this amp, not using Rg.
So most likely this component is just missing and should be inserted.
Hans
Hans,
if you look in the Data Sheet of the 1512 it is possible. The original design of the SAC gamma has also no Rg in the input !!!
I am trying to get the schematics, because its hard to draw the whole schematic.
regards
Dieter
if you look in the Data Sheet of the 1512 it is possible. The original design of the SAC gamma has also no Rg in the input !!!
I am trying to get the schematics, because its hard to draw the whole schematic.
regards
Dieter
So the SAC Gamma is working again (some cold solder pads), but I think aTHAt 1510 should make a better job vor MC-Input
Dieter
Dieter
Hans,
I have got the schematics, yes there is a RG (526 Ohm), but still some strange behavior...
Regards
Dieter
I have got the schematics, yes there is a RG (526 Ohm), but still some strange behavior...
Regards
Dieter
I had a brief look at the Sac.
It has an SE MM input and a differential MC input.
For the latter input a That1512 has been used as a step up amp, starting with 20dB gain (Rg=526) going in 2 dB adjustable steps to 28 dB by paralleling resistors to Rg.
So what strange behaviour do you experience, and which input are you using, MM or MC. When using the MC input, have you checked that the 4 cart wires are fully floating and not connected to any ground ?
Hans
It has an SE MM input and a differential MC input.
For the latter input a That1512 has been used as a step up amp, starting with 20dB gain (Rg=526) going in 2 dB adjustable steps to 28 dB by paralleling resistors to Rg.
So what strange behaviour do you experience, and which input are you using, MM or MC. When using the MC input, have you checked that the 4 cart wires are fully floating and not connected to any ground ?
Hans
There is no reason to do so, as long as I have no idea what your problems are.
MC/MM , one channel/both channels, supply voltage O.K. ?????
Hans
Hans,
there is a bad noise (both MC/MM) on both channels. Supply Voltage is fine (14,8 V). The owner gave me a XLR to Cinch adapter. If I plug this adapter in MC mode in, then the preamp is quit, but if I connect the cartridge, the noise comes again. Very strange
Regards
Dieter
there is a bad noise (both MC/MM) on both channels. Supply Voltage is fine (14,8 V). The owner gave me a XLR to Cinch adapter. If I plug this adapter in MC mode in, then the preamp is quit, but if I connect the cartridge, the noise comes again. Very strange
Regards
Dieter
Question 1: That sounds as if the the 4 wires of the cartridge are not floating as they should be in this differential setup. The TT gnd should be separetely connected to the preamp chassis. This can all be achieved with xlr cables with 3 wires or with Cinch cables and a separare gnd wire, but under no condition may any of the 4 cart wires be connected to gnd.
Question 2: Is the preamp in MC mode also quiet when is not connected to a TT and without rca to xlr adapter ?
Hans
Hans,
Pin 2 an3 of the XLR goes direct (via 2,2uH) to the THAT1512. Pin 1 goes to the ground on the PCB. The XLR is a print version.
The preamp makes also in MC mode bad noise...
Thanks
Dieter
Pin 2 an3 of the XLR goes direct (via 2,2uH) to the THAT1512. Pin 1 goes to the ground on the PCB. The XLR is a print version.
The preamp makes also in MC mode bad noise...
Thanks
Dieter
Dieter,
Let's structure the info a bit.
So whith nothing connected you have a lot of noise. Is this in the MC setting or also in the MM setting and is it the same for left and right channel ?
What happens with this noise when you short cicuit the MC input in the MC mode and do the same in the MM mode.
Input impedance at the MC input should be 1k ohm, do you measure this ?
And what is the impedance from pin 2 and pin 3 to Gnd ?
When you connect pin 2 or pin 3 to Gnd, does the noise stop?
Hans
Hans,
tomorrow we have public holiday, so I have more time. Input Impedance is 1k, yes. Whe I shorten pin 3 XLR to ground, the noise is gone.
Dieter
tomorrow we have public holiday, so I have more time. Input Impedance is 1k, yes. Whe I shorten pin 3 XLR to ground, the noise is gone.
Dieter
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O.k. with the circuit diagram at hand I can be more specific.
From your previous answer telling that noise is gone when connecting pin3 to Gnd, I conclude 1) that only one channel is causing problems Y/N ? and 2) that when switching to MM the noise is also gone Y/N ?
If both true, this reduces the problem to the That1512 stage.
Do you measure 1K between pin3 and Gnd, if not the resistor is defective if yes, the That1512 is broken and should be replaced.
Hans
From your previous answer telling that noise is gone when connecting pin3 to Gnd, I conclude 1) that only one channel is causing problems Y/N ? and 2) that when switching to MM the noise is also gone Y/N ?
If both true, this reduces the problem to the That1512 stage.
Do you measure 1K between pin3 and Gnd, if not the resistor is defective if yes, the That1512 is broken and should be replaced.
Hans
Hans,
the input Rs are Ok (1k). What I have found out:
- MC Mode Pin 3 XLR to ground (with the XLr Cinch adapter) absolute quite
- MC Mode Pin 3 XLR to ground a cartridge is connected noise !!!
- MM Mode input open very bad noise, cartridge connected, very quite !!!
and that on both channels !!! I don't think the THAT 1512 is the problem, becaus in MM mode the noise sound very bad.
Regards
Dieter
the input Rs are Ok (1k). What I have found out:
- MC Mode Pin 3 XLR to ground (with the XLr Cinch adapter) absolute quite
- MC Mode Pin 3 XLR to ground a cartridge is connected noise !!!
- MM Mode input open very bad noise, cartridge connected, very quite !!!
and that on both channels !!! I don't think the THAT 1512 is the problem, becaus in MM mode the noise sound very bad.
Regards
Dieter
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