Hi guys, I need your suggestions:
I have a turntable with a MC capsule (Technics SL-M1, AT OC9/III) and I want to connect it to 2 pre-phonos, one tubes (EAR 834p) and other SS (Pass XP-15).
I thought that a RCA switcher (tc-7240) could do the job (turntable -> switch -> ph1 & ph2 ->prea-amp (passive) ->amps), but not, I got a heavy "hum" when I take this way. Mainly from XP-15, seams that EAR is more "tolerant".
Off course I can connect and disconnect, but I think that soon I will break something.
Any ideas?
Thank you,
I have a turntable with a MC capsule (Technics SL-M1, AT OC9/III) and I want to connect it to 2 pre-phonos, one tubes (EAR 834p) and other SS (Pass XP-15).
I thought that a RCA switcher (tc-7240) could do the job (turntable -> switch -> ph1 & ph2 ->prea-amp (passive) ->amps), but not, I got a heavy "hum" when I take this way. Mainly from XP-15, seams that EAR is more "tolerant".
Off course I can connect and disconnect, but I think that soon I will break something.
Any ideas?
Thank you,
There are probably connections between the two output grounds. You may be able to adapt it.
Both the hot and ground of each signal should be switched, for both the inputs and outputs.
Then it can't cause hum. Post a photo of the inside.
Both the hot and ground of each signal should be switched, for both the inputs and outputs.
Then it can't cause hum. Post a photo of the inside.
This is the switch:
An externally hosted image should be here but it was not working when we last tested it.
Despite the gold plate jacks and professional lettering, all RCA jacks outer rings are connected to case. Only the centers are switched. This is ****. You could have bought a TV source selector for $12 that did that.
You could add a wafer and switch the rings, but they have carfully built the circuit board where you can't do that. You would also have to insulate the rings from case with o-rings or cardboard cutouts.
Probably the best thing to do is put a riaa 50x gain stage in each turntable. You can get 5532 circuits on boards from ebay that look like they may be pretty good. Then mix with 10k resistors to a 2x gain op amp circuit with volume control in a central box. I like 33078, they are cheap, available, quiet and accurate. But 5532 is not improbable for the 2x mixer application, either. Keep your power supplies away from the high gain boards with a wall transformer. You can split +15 to +30 dc from a wall transformer with zeners or regulators & capacitors in the gain stage area, since there will be no hum in dc regulation.
You could add a wafer and switch the rings, but they have carfully built the circuit board where you can't do that. You would also have to insulate the rings from case with o-rings or cardboard cutouts.
Probably the best thing to do is put a riaa 50x gain stage in each turntable. You can get 5532 circuits on boards from ebay that look like they may be pretty good. Then mix with 10k resistors to a 2x gain op amp circuit with volume control in a central box. I like 33078, they are cheap, available, quiet and accurate. But 5532 is not improbable for the 2x mixer application, either. Keep your power supplies away from the high gain boards with a wall transformer. You can split +15 to +30 dc from a wall transformer with zeners or regulators & capacitors in the gain stage area, since there will be no hum in dc regulation.
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If you want to switch the cartridge output from one phono preamp to another, a special shielded box
would have to be built. It would switch both hot and cold of each output (use only when the system is off).
Even so, there could be hum/noise added, especially with the medium output MC cartridge being used.
Is there someone local who could build this for you?
would have to be built. It would switch both hot and cold of each output (use only when the system is off).
Even so, there could be hum/noise added, especially with the medium output MC cartridge being used.
Is there someone local who could build this for you?
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Normally a phono lead has two shielded cables and a ground-wire. All probably need switching to avoid ground issues, although the grounding wire might be sharable - this is an easy experiment to do.
@indianajo , I'm not very comfortable with add another step to the chain, I'm using a TVC passive pre-amp to try to arrive with the most posible clean signal to amp. Thank you.
@rayma , I have a amp-speaker switch too, that change both connection for both cables, I will try this week-end and let you know. Thank you.
@ Mark Tillotson , I've tried changing ground cable, the humm is lower but still very noisy. Thank you.
Sorry for the delay, but I'm one of the fortunates with job in this crisis and has been hard to have time.
Thank you for your help, I'll make some test and share the results.
Be safe!
@rayma , I have a amp-speaker switch too, that change both connection for both cables, I will try this week-end and let you know. Thank you.
@ Mark Tillotson , I've tried changing ground cable, the humm is lower but still very noisy. Thank you.
Sorry for the delay, but I'm one of the fortunates with job in this crisis and has been hard to have time.
Thank you for your help, I'll make some test and share the results.
Be safe!
Thank you for the large interior photos. I just received the TC-7240 and it suits my needs exactly. I have a Yamaha home theater amp with no phono input, and an EICO HF-81 tube amp that has a great phono input. I have 6 turntables/changers, none of which have preamps built in because they are older 60's and 70's units like a Dual 1219, a Garrard SL-95 in the large wood "chairside plinth", a BSR 810X, a NEAT P-68H, an ADC Accutrac 4000, etc. I considered building a preamp into the Garrard base so that I could take the unit to vinyl parties but no one around here does that anymore.
There appears to be enough room inside the TC-7240 to build-in four small RIAA preamps. The box is advertised as a "Line-Level" switch, aka a 1-V P-P switch. With four turntables connected, even with a heavy ground system, you almost certainly will get some hum. I searched everywhere for a box that had preamps already built-in, to no avail. At least not with 4 or 5 phono inputs.
There are many commercially-available preamp boards but they appear to be about 2 inches square, so they won't fit. I'm thinking about building four amps on a long narrow phenolic board.
There appears to be enough room inside the TC-7240 to build-in four small RIAA preamps. The box is advertised as a "Line-Level" switch, aka a 1-V P-P switch. With four turntables connected, even with a heavy ground system, you almost certainly will get some hum. I searched everywhere for a box that had preamps already built-in, to no avail. At least not with 4 or 5 phono inputs.
There are many commercially-available preamp boards but they appear to be about 2 inches square, so they won't fit. I'm thinking about building four amps on a long narrow phenolic board.
Actually, I just performed some tests with my new TC-7240. With everything grounded to the 7240, the amps, and a pre-amp for the Yamaha theater amp, the only phono with any discernable hum is the Garrard 95B, and I think I can solve that with a heaver ground cable. Interestingly, the quietest table, except from the idler drive rumble, has always been the NEAT, and it doesn't even have a ground cable.
When I was a teen, I became infatuated with the Darlington-pair series of transistors offered by Motorola, and I built FM antenna preamps on phenolic boards in metal cases that went between the antenna cable and radios in cars. It's an easy construction technique. I bet a small RIAA amps with fixed gain could be built up on such a concept. The output might not be 1V P-P exactly, but the gain at the input would dramatically reduce the hum.
When I was a teen, I became infatuated with the Darlington-pair series of transistors offered by Motorola, and I built FM antenna preamps on phenolic boards in metal cases that went between the antenna cable and radios in cars. It's an easy construction technique. I bet a small RIAA amps with fixed gain could be built up on such a concept. The output might not be 1V P-P exactly, but the gain at the input would dramatically reduce the hum.