Hi!
As I'm new here, please let me know if I break any rules of the forums.
I have been bitten by the shellac bug and started to collect 78s, hot jazz to be specific. I'm amazed by how much better they sound than many of the digitizations available. You can actually hear the individual instruments, not just a generic piano. And you can feel the soul of Hodges blowing into the saxophone.
So far I have been using an inexpensive TCC TC-778 preamp, but I am now setting up a second system for playback and am now looking into better alternatives. I am considering the alternative of buying a TC-750 and modify the RIAA network to be switchable. Comparing the TC-778 to the reverse engineered schematic of the TC-750 I found that it is very similar (some resistors and capacitors differ). There is a switch between C4 and Q4 to switch it in and out of the eq network, removing the high frequency pole for 78 playback.
Is it feasible to take the PCB and put it in a larger enclosure with a switch to switch in other capacitors for C4 and C5 to get different eq curves? Could the network be broken into a hf and a lf part and put somewhere else in the circuit, for example putting the hf part after Q3?
I also noticed that the "50 hz" pole seems to be at 38 Hz in the schematic, and at 35 Hz in the TC-778 (R6 is replaced by a 379k resistor). Is there something I'm missing here, or is this normal?
If the TC-750 is unsuitable for this kind of modification, is there some other inexpensive phono amp (available in europe preferably) that you can recommend for modification? I'd prefer not to build something from scratch.
Cheers
Peter
As I'm new here, please let me know if I break any rules of the forums.
I have been bitten by the shellac bug and started to collect 78s, hot jazz to be specific. I'm amazed by how much better they sound than many of the digitizations available. You can actually hear the individual instruments, not just a generic piano. And you can feel the soul of Hodges blowing into the saxophone.
So far I have been using an inexpensive TCC TC-778 preamp, but I am now setting up a second system for playback and am now looking into better alternatives. I am considering the alternative of buying a TC-750 and modify the RIAA network to be switchable. Comparing the TC-778 to the reverse engineered schematic of the TC-750 I found that it is very similar (some resistors and capacitors differ). There is a switch between C4 and Q4 to switch it in and out of the eq network, removing the high frequency pole for 78 playback.
Is it feasible to take the PCB and put it in a larger enclosure with a switch to switch in other capacitors for C4 and C5 to get different eq curves? Could the network be broken into a hf and a lf part and put somewhere else in the circuit, for example putting the hf part after Q3?
I also noticed that the "50 hz" pole seems to be at 38 Hz in the schematic, and at 35 Hz in the TC-778 (R6 is replaced by a 379k resistor). Is there something I'm missing here, or is this normal?
If the TC-750 is unsuitable for this kind of modification, is there some other inexpensive phono amp (available in europe preferably) that you can recommend for modification? I'd prefer not to build something from scratch.
Cheers
Peter
Attachments
See this thread that covered the various equalization curves of different 78 products, and a circuit to play them back properly:
http://www.diyaudio.com/forums/anal...te-legacy-pre-amplifer-all-78s-early-lps.html
I build circuits for discrete transistors with point to point wire on Nema CE laminate which mcmaster calls garolite. Modifying an existing circuit board might be easier, but it might not. And it is more expensive.
In the US I use MPSA56/06 or MPS8099/8098 for experiments. At 12 v 2n3904 is appropriate.
Circuits with DIL packages like the op amp in the thread, I build on project boards from mcmelectronics.com. About $2 each. I believe there is an equivalent available in France but I didn't preserve the link. I use phosphor bronze plate sockets from TE connectivity. Brass and tin oxidize faster.
http://www.diyaudio.com/forums/anal...te-legacy-pre-amplifer-all-78s-early-lps.html
I build circuits for discrete transistors with point to point wire on Nema CE laminate which mcmaster calls garolite. Modifying an existing circuit board might be easier, but it might not. And it is more expensive.
In the US I use MPSA56/06 or MPS8099/8098 for experiments. At 12 v 2n3904 is appropriate.
Circuits with DIL packages like the op amp in the thread, I build on project boards from mcmelectronics.com. About $2 each. I believe there is an equivalent available in France but I didn't preserve the link. I use phosphor bronze plate sockets from TE connectivity. Brass and tin oxidize faster.
Last edited:
rayma: Thanks. I have read that thread, and while informative I think it mostly covers finished (expensive) products and complete diy builds?
indianajo: Thank you for your input. That looks like a relatively simple build if I choose to build from scratch instead of modifying an existing preamp. My thinking was that I would feel more comfortable doing a modification and verify by calculation and listening if I slightly modify an existing product, whereas if I build from scratch I should probably do some more ambitious measuring. Also, would it be possible to modify your design for a variable bass shelf? I probably should work out the calculations by myself, but if you have a link to the derivation if the component values in the eq network I wouldn't mind being lazy.
indianajo: Thank you for your input. That looks like a relatively simple build if I choose to build from scratch instead of modifying an existing preamp. My thinking was that I would feel more comfortable doing a modification and verify by calculation and listening if I slightly modify an existing product, whereas if I build from scratch I should probably do some more ambitious measuring. Also, would it be possible to modify your design for a variable bass shelf? I probably should work out the calculations by myself, but if you have a link to the derivation if the component values in the eq network I wouldn't mind being lazy.
I must have been paying too little attention to that left column with the username. I'll try asking in Monty78pig's thread instead.- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.