Greetings!
It would be nice to have such a feature on my next DIY phono preamp build. The current circuit has an input section that allows selection of load and gain settings to adapt to MM or MC carts, through DIP switch SW1 :
Could the proposed selector be implemented by duplicating that section and add DPDT switching in front of the input transistors like shown below?
The switching itself would probably be through signal relays. I would guess a break-before-make switching is necessary to avoid momentarily connecting both carts and their loads in parallel during switching. The 10K resistor in the dotted circle is an addition to avoid having the input stage emitters floating during switching while not affecting gain much during normal operation.
Is there danger of undesirable thumps/signal spikes from the switching of the carts' coil ? If so I could split the 47K base resistor to ground into two 96K in parallel, one on the bases and one at each of the inputs so that the carts are always presented with a load. Does that make any sense?
Thanks in advance,
- Joris
It would be nice to have such a feature on my next DIY phono preamp build. The current circuit has an input section that allows selection of load and gain settings to adapt to MM or MC carts, through DIP switch SW1 :
Could the proposed selector be implemented by duplicating that section and add DPDT switching in front of the input transistors like shown below?
The switching itself would probably be through signal relays. I would guess a break-before-make switching is necessary to avoid momentarily connecting both carts and their loads in parallel during switching. The 10K resistor in the dotted circle is an addition to avoid having the input stage emitters floating during switching while not affecting gain much during normal operation.
Is there danger of undesirable thumps/signal spikes from the switching of the carts' coil ? If so I could split the 47K base resistor to ground into two 96K in parallel, one on the bases and one at each of the inputs so that the carts are always presented with a load. Does that make any sense?
Thanks in advance,
- Joris
You are bound to hear a bang during switching due to the difference between the NPN and PNP base currents having to flow through a different resistance. When you use relays anyway, you could try to come up with a control circuit that first mutes the amplifier output with a relay at the output, then does the switching, then waits for a second or two and then deactivates the mute. Either that or do something with AC coupling at the inputs to prevent DC level jumps.
Edit: as you also switch gains, the latter proposal is not likely to work.
Edit: as you also switch gains, the latter proposal is not likely to work.
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Thanks Marcel for your reply. This is perfectly possible to do with a basic microcontroller circuit. The muting could also be used by itself with a front panel button as a nice feature to have. I'd wire it so it is normally muted beacuse the current build produces a substantial thump when powered down.
That would prevent damaging downstream equipment, but could this overload damage parts in the phono stage itself? I guess it could be assimilated to a big "pop" from playing a LP?
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Thanks jenimitso for your reply. This design started as an execise to clone an existing amp that I have; in a future design I may check the opportunity of a FET. That being said the current noise through the input transistors has been optimized with the help of MarcelvdG in this post. I don't have any measuring equipment to evaluate the noise performance, but the end result is quite listenable on both MC and MM carts.
The circuit is rather complex and intensive to build, I'd rather have a selector in front since I only listen to one turntable at a time. I don't have a dual tonearm setup and don't plan to own one any time soon.
I want to switch between two input circuits where each can be indepedently configured for MM or MC (see post #1) by DIP switches. The input circuits incorporate load capacitor(s) and the required resistors to alter the gain of the following input stage transistors for this purpose.
For example I currently have two turntables in a vinyl recording setup, one with MM cart the other MC, with two corresponding phono preamps. I'd like the same preamp handle both, as I record only one LP at a time.
For example I currently have two turntables in a vinyl recording setup, one with MM cart the other MC, with two corresponding phono preamps. I'd like the same preamp handle both, as I record only one LP at a time.
Many preamps that handle MC and MM have separate inputs for MC and MM cartridges, the natural place to switch between them is at the input to the MM preamp (the MC head end typically feeds in to this point).