Revisiting the question of how to wire the head(s)

PS2500 · 4152

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Offline PS2500

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on: August 08, 2021, 10:13:02 PM
In this subforum, Doc B answered a question of mine about keeping the internal preamp of a tape deck available by suggesting cutting the wires between the head and the preamp. Reading it over, I thought I got it, but actually no.

Instead of a switch you could cut the internal cables. Run the part of the cables left connected to the internal preamp to a second pair of RCA jacks, as well as running the end left connected to the playback head to a pair of RCA jacks. Then you can use an interconnect to connect the head either to an outboard playback preamp or loop it back into the internal preamp. That's basically how I use my Nagra T, with different connectors.

I've been thinking about how this would work in a Technics RS1500. There are two line outs available on the back of the deck, as well as a line in and a "through out". I assume the through out is for directly outputting a source that you might be recording from to an amplifier. If so, I don't really need it and could repurpose that connector.

So while I'm still not totally clear about Doc B's suggestion, I think a Technics deck could have 3 RCA connections on the back that can be rewired (keeping the 4th, the line in, for recording, of course). One for direct connection from the playback head, and from there over interconnect to an Eros amp. One for direct connection to the OUT side of the internal preamp board. And then one to the IN side of the internal preamp board?

Do I have that right? If so, then the back of the deck could have a "playback head out", an "internal preamp head out" and an "internal preamp in" connector? And then to use the internal amp, just use an interconnect to bridge playback head out and internal preamp in?

Finally, is it possible to have both the 4 tr head and the 2 tr head wired directly to the output at the back of the deck?
« Last Edit: August 08, 2021, 10:47:50 PM by PS2500 »



Offline Doc B.

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Reply #1 on: August 09, 2021, 06:57:00 AM
What is an internal preamp head out?

Leave the line input jack alone. Leave the line output 1 jack alone. Cut the wires to the line output 2 jack and connect the internal play head cables coming from the head to those jacks instead of the internal preamp. Cut the wires to the throughout jacks and connect those jacks to the input of the internal playback preamp. Then you can run interconnects from the head out jacks (labeled line output 2) to either an Eros or to the internal playback preamp jacks (labeled throughout).

Both play heads play thru the same internal head cables going to the preamp. They meet at the switch in the head block, before those cables.

I will suggest getting an RS1500 service manual and studying the schematics.

Dan "Doc B." Schmalle
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Offline PS2500

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Reply #2 on: August 10, 2021, 02:18:35 AM
Quote
What is an internal preamp head out?

Yes, sorry, I meant to say internal preamp out. Thanks for the explanation, I've got it now. Will definitely be looking at the schematic of course, before I do anything with the wires. I originally had this project in mind for a different machine but now have a Technics RS1500 that I would prefer to use instead. I suspended the build itself several times while I had other projects on the go, so it's been glacially slow, but I restarted recently and want to carry through to the end in the near future.   



Offline timmieec

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Reply #3 on: January 17, 2024, 09:51:38 AM
Yes, sorry, I meant to say internal preamp out. Thanks for the explanation, I've got it now. Will definitely be looking at the schematic of course, before I do anything with the wires. I originally had this project in mind for a different machine but now have a Technics RS1500 that I would prefer to use instead. I suspended the build itself several times while I had other projects on the go, so it's been glacially slow, but I restarted recently and want to carry through to the end in the near future.   

Hi @PS2500!

Were you able to go through with the rewiring to enable running interconnects from the head out jacks to the internal playback preamp jacks?

Timothy



Offline PS2500

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Reply #4 on: January 19, 2024, 04:57:26 PM
Hi Timmiec,

Not yet, but thanks for the kick in the pants. The project has been stops and starts all the way, as I set it aside to focus on other things, both related and less related. I recapped and calibrated the tape deck I want to use it with, which all took a while. Then various turntable and other audio projects took precedence, and that all fits around other interests, as well as travel plans...months pass, then years pass!

I've been meaning to get back to Paul's suggestion about testing the amp with a phone to make sure that the issue I was getting is in the deck and not the amp. If I do make some time this weekend, I'll try it. It's not the procedure that's complicated, obviously enough. Just me and my sloth that's complicating it.
« Last Edit: January 19, 2024, 06:01:54 PM by PS2500 »



Offline timmieec

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Reply #5 on: February 02, 2024, 07:24:48 PM
Ahaha I totally understand what you mean. I myself have been meaning to make this modification so I can bypass the RS1500's internal preamp but life happens and the mod work gets pushed out.



Offline PS2500

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Reply #6 on: February 04, 2024, 10:25:22 PM
Okay, so I finally did the test with the phone, and I have audio (albeit bassy, as Paul said it would be).That's good, it must mean I constructed the amp correctly. So I'll have to troubleshoot what I did wrong with my RS1500 head connection. If that becomes too much of a problem, I have a couple of other tape decks I could try, and perhaps I'll have better luck with one of those. For now, I think I must have just shorted the audio signal from the RS1500, so I got nothing but a loud hum. It's a good while ago now, so I don't remember in clear detail, but it happened whether or not anything was playing, and no audio coming through at all. As my other projects are completed for the moment, I might have some time to spend on fixing that.




Offline Doc B.

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Reply #7 on: February 06, 2024, 07:24:33 AM
Sounds like a lifted signal ground to me.

Dan "Doc B." Schmalle
President For Life
Bottlehead Corp.


Offline PS2500

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Reply #8 on: February 23, 2024, 02:55:33 AM
Thanks.

I'll hunt it down eventually. I keep getting waylaid by other projects: some tape deck-related, some turntable related. Most of them are nearing completion now, so I can go back to the Eros soon, I hope.