Page 15 of 16

Re: Phono Transfer System Construction Information

Posted: Sat Jul 27, 2019 5:48 pm
by billshurv
Exactly. I have some kelvin probes I want to test out anyway :)

Re: Phono Transfer System Construction Information

Posted: Wed Aug 14, 2019 3:32 am
by billshurv
I have actually started soldering. Based on some previous bad experiences I now never solder after midnight so limited time as eldests is in the room i use as a lab so everything has to be packed away each night. I need a bigger house...

Re: Phono Transfer System Construction Information

Posted: Wed Aug 14, 2019 11:48 am
by mediatechnology
Which board are you building?

Re: Phono Transfer System Construction Information

Posted: Wed Aug 14, 2019 5:06 pm
by billshurv
I'm starting with the Flat MM boards, then I'll do the RIAA switchers then the two MC boards. Once both sets are tested then I send one set to the low countries for a friend and I can start playing with my MM cartridges.

Re: Phono Transfer System Construction Information

Posted: Wed Aug 14, 2019 6:29 pm
by mediatechnology
I owe you some test instructions for the MC preamp board.
I need to get back on that project.

Re: Phono Transfer System Construction Information

Posted: Thu Aug 15, 2019 3:25 am
by billshurv
Sure I can work that out!

Re: Phono Transfer System Construction Information

Posted: Wed Oct 30, 2019 6:06 pm
by billshurv
OK easy bit is all done. Now working on the customisations for one of the sets and trying to guestimate the gains required. The bit I'm having trouble on is relative levels between Flat and RIAA outputs. I want to think they should be same at around -23dBFS for a 0dB reference cut, but keep thinking I'm missing something.

Re: Phono Transfer System Construction Information

Posted: Wed Oct 30, 2019 7:05 pm
by mediatechnology
Sounds like progress!

-23 dBFS (flat) is my target for 1 kHz 5 cm/s.

The RIAA filter has a -2.25 dB insertion loss at 1 kHz which is usually made up at VR3/VR4.
If the RIAA is set for unity gain overall the subjective levels between flat and EQ'd are pretty close.

There's 6 dB of gain built in RIAA EQ monitor due to the THAT1646 and 6 dB in the cross-coupled output of the MM and MC preamps.
The INA front-end gain should thus be 12 dB less than the overall required gain.
The INA gain is 1+[(2*Rfb)/Rg] for both the MM and MC preamps.

Re: Phono Transfer System Construction Information

Posted: Thu Oct 31, 2019 4:52 am
by billshurv
Wife is away for half term. But have put down the soldering iron whilst I measure twice and cut once on the build I am doing for a friend. Next step is working out positionng of the 26 relays needed!

Re: Phono Transfer System Construction Information

Posted: Wed Nov 20, 2019 6:34 pm
by billshurv
So in department of 'measure twice, cut once' to cover the range of MCs in use I need low/high gain settings approx 8dB apart to keep the flat output within a good range on one build. The Flat output will go into a Behringer DEQ with no input attenuator. I have gain control on the RIAA path so that is fine. After dallying with the idea of switching gain on the fly on the MC board and deciding against it I'm looking at the phono switcher board and can see at least 3 places I could do it and was about to commit to solder when I realised I had not taken into account possibly switching glitches. Right now I'm seriously considering wimping out and putting a pot in the HPF insert points as the least risk as this unit will end up with someone else and I don't want to have to remotely debug it. For my unit I'll have time to mess around switching in a parallel resistor on J2.