I have two IMG STA-1000D with pascal S-PRO modules.
Some time in the future I would like to skip the nuid in preamp section, and build a new one. The build in preamp uses ST TIP122 and NJM4558LD opamps. I am not familiar with the quality of the preamp section, but it could be fun to build a better one, and skip the potentiometers.
The modules have a 2.2 kohm input impedance, so I guess the preamp should have low Zout. I haven't seen much data for Zout or preamps ability to drive such a heavy load.
This one is pretty close, but I would some different opportunities.
Minimalist Discrete Hi-Fi Preamp
I don't mind if it's based on opamps.
Some time in the future I would like to skip the nuid in preamp section, and build a new one. The build in preamp uses ST TIP122 and NJM4558LD opamps. I am not familiar with the quality of the preamp section, but it could be fun to build a better one, and skip the potentiometers.
The modules have a 2.2 kohm input impedance, so I guess the preamp should have low Zout. I haven't seen much data for Zout or preamps ability to drive such a heavy load.
This one is pretty close, but I would some different opportunities.
Minimalist Discrete Hi-Fi Preamp
I don't mind if it's based on opamps.
TIP122's sound more like regulator transistors. The 4558 (if used for audio) would be the first thing to look at replacing.
We would really need to see a circuit to make a judgement. The opamp could be part of the power supply.
2k2 input impedance is low, but any opamp (even the TL071/2 type) would have no difficulty fully driving that.
We would really need to see a circuit to make a judgement. The opamp could be part of the power supply.
2k2 input impedance is low, but any opamp (even the TL071/2 type) would have no difficulty fully driving that.
Yes TIP122 is regulator. I don't have a circuit, but I do have a picture.
There are four NJM4558LD opamps
There are four NJM4558LD opamps
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Single in line opamps 🙁
That makes swapping them more difficult. You would need adapter boards (readily available though) to use either standard DIP packages or SMD type.
Those in the top half of the board are definitely audio related and would be a definite worthwhile swap.
If you could get hold of some JRC2068 or NJM2068 (which are available in SIP outline) then that would get you most of the way there. The 2068 is a very good device, similar to the NE5532 in many ways.
That makes swapping them more difficult. You would need adapter boards (readily available though) to use either standard DIP packages or SMD type.
Those in the top half of the board are definitely audio related and would be a definite worthwhile swap.
If you could get hold of some JRC2068 or NJM2068 (which are available in SIP outline) then that would get you most of the way there. The 2068 is a very good device, similar to the NE5532 in many ways.
I would prefer not to modify the existing board but bypass it. I have the pinouts from the S-PRO module, so I can take a well proven preamp/buffer board and connect directly to the S-PRO module.
For another option, the trusty NJM4556A was designed basically as a "4558 with oomph", and it's old enough to have been made in SIP (AL suffix) varieties as well. Could be the ticket here, assuming supply voltages are within spec (LD points towards old parts rated for +/-21 V, 4556A is +/-15 V max like a lot of parts - this circuit seems to be single supply, so +30 V max). I'd recommend finding out which parts are actually driving the outputs, replacing all of them may put undue stress on the power supply as the 4556A draws quite a bit more current (9 mA typ vs. 3.5 mA). It's normally used as a headphone driver and rated for driving 150 ohms at max supply. It's also a low-Ib part.
You can also look for L suffix NJM4560 and NJM4565 parts, which are similar to the 4558 but are faster and have increasingly better output driving abilities at less inordinate current draw (about 4.5 mA). Mouser also carries these.
NJM2068 is a medium-Ib, low-noise part, and I'd be worried about it having too much input capacitance to be a drop-in replacement for a high-impedance circuit. Its driving abilities are OK but not anything special, 2k2 is about the lower limit at high levels. Originally being geared towards higher-gain applications (think mic or tape head preamp), it uses 2nd-order compensation to squeeze out more GBW at audio frequencies. It's a relatively modern part, same generation as NJM4580 if memory serves.
You can also look for L suffix NJM4560 and NJM4565 parts, which are similar to the 4558 but are faster and have increasingly better output driving abilities at less inordinate current draw (about 4.5 mA). Mouser also carries these.
NJM2068 is a medium-Ib, low-noise part, and I'd be worried about it having too much input capacitance to be a drop-in replacement for a high-impedance circuit. Its driving abilities are OK but not anything special, 2k2 is about the lower limit at high levels. Originally being geared towards higher-gain applications (think mic or tape head preamp), it uses 2nd-order compensation to squeeze out more GBW at audio frequencies. It's a relatively modern part, same generation as NJM4580 if memory serves.
Hi Kjeldsen
Did you find a good solution where you bypassed the input module? I have the same amp and would very much appreciate to hear what you ended up doing 🙂
Did you find a good solution where you bypassed the input module? I have the same amp and would very much appreciate to hear what you ended up doing 🙂
I will just find a good preamp module. I still build and test new speakers. But I have got another amp, so I just need to prioritize the amp rebuild.
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