Help me wire up my SUT correctly (Tamura TKS-27)

[rongon]

I'm not comfortable with the grounding, etc. of MC step up trannies for phono cartridges.

I've attached a little schematic of the SUT in question. They're mounted in an ugly little electrical enclosure, steel chassis. The box has a pair of RCA jacks for input. The input jacks are floating from the chassis. I will be hardwiring the outputs to the interconnect cable, coming straight from the enclosure.

I want to wire up the SUT with single-ended input, using the 300 ohm primary (lugs 1 and 3). Secondary lugs are 4 and 6, 100k ohms. Secondary will also be single-ended.

My dumb questions:

- For unbalanced input, should lug 3 be grounded? If yes, where? Directly to the chassis? Or should the single-ended input float, leaving only the ground wire from the turntable grounded to the chassis?

- Lug 5 is a shield for the secondary. I'm assuming it should be grounded to the chassis?

- For unbalanced output, should lugs 5 and 6 be shorted and grounded? Lug 4 would then be the output signal 'hot', 5+6 would be output ground?


I think that's it. Any help would be greatly appreciated.

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[rongon]

According to this schematic...

...if I'm reading it correctly, for the primary, I would ground the input shield and lug 3 (0 ohms), and the input 'hot' would be lug 1 (300 ohms).

I would leave the secondary floating by grounding lug 5, while lug 4 would be signal 'hot' and lug 6 would be signal 'shield' (not grounded until the RIAA preamp).

Maybe I'll just try it that way and see what happens.

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[SY]

Don't ground the primary. And if you can convert the cartridge-to-transformer wiring to balanced with a shield, that's even better (ground the shield). Secondary grounds from pin 6.

 

[rongon]

Thanks SY. I think this is what you meant (single-ended input, drawing attached). Did I get it right?

 

[SY]

Yep!

If you use twisted wires for cartridge hot and cold, connect to the primary, and then ground the shield, you'll have an even better interface.

 

[rongon]

Unfortunately, I'm using a Technics SL-1200mk2 'table with its stock tonearm wiring, and I'm not sure how the tonearm is wired. According to this picture...

...it looks like the cartridge cold conductors are grounded. But I could be wrong about that. At any rate, thanks for the help!

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[rongon]

Well, I thought I should let you know how this all ended up. I have to abandon the project for now. Two things did me in. One, the Soundsmith Gold cartridge (modified DL103R, 14.4 grams!) is way too heavy for my tonearm, and two, my phono stage is too sensitive for the Tamura step-up transformer (too high a step-up ratio). In order to make this all work, I'd need a new turntable/tonearm combo, and I'd need to alter the RIAA amp or make a new one. That's more new stuff than I had in mind.

For now, I'm happy enough with my current modest setup, but this was a great learning experience for me. Thanks for the help.

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[vinylkid58]

Well, I thought I should let you know how this all ended up. I have to abandon the project for now. Two things did me in. One, the Soundsmith Gold cartridge (modified DL103R, 14.4 grams!) is way too heavy for my tonearm, and two, my phono stage is too sensitive for the Tamura step-up transformer (too high a step-up ratio). In order to make this all work, I'd need a new turntable/tonearm combo, and I'd need to alter the RIAA amp or make a new one. That's more new stuff than I had in mind.

You can add mass to the stock Technics c-weight. Or, swap the arm out for a Jelco 750d.

Technics SL-1200 mods

Seems odd that you have too much gain. Is the Tamura step-up more than 10:1?

jeff

 

[rongon]

I think the Tamura is more than 10:1, and I also know that my preamp has more than the usual 40dB of gain. I think it's more like 50dB. I designed it that way to match the output from my DL110 to that of a CD player. The combination of the Soundsmith Gold, Tamura SUT and my 'hot' preamp is just too much. But the electronic stuff is easy compared to the mechanical requirements of the cartridge and tonearm.

I have the extra 10g screw-in weight already installed on the stock counterweight. I'd have to put some kind of non-standard extra weight on there. I guess I could put a layer of something on the screw in weight to add a bit more weight.

I'll read up on the Jelco 750D option. I guess a Rega RB300 tonearm can't handle a 14.4 gram cartridge?


Wait -- That brass pipe weight looks like it would work. Where can I find details on that? Is that glued to the KAB add-on weight? I don't get it...


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[vinylkid58]

I guess I could put a layer of something on the screw in weight to add a bit more weight.

You could try adding Duct Seal around the KAB weight until you reach the correct balance point. Then, take it off and weigh it, which would give you a reference point for a more permanent solution.

I'll read up on the Jelco 750D option. I guess a Rega RB300 tonearm can't handle a 14.4 gram cartridge?

For the Rega, you'd have to get an aftermarket "heavyweight" . I went the Jelco route because of the removable head-shell. I found mounting a large-ish cartridge on a Rega to be a real PITA.

Wait -- That brass pipe weight looks like it would work. Where can I find details on that? Is that glued to the KAB add-on weight? I don't get it...

I'm thinkin' it came from the hardware store, home despot maybe? Don't know how they mounted it.

jeff

 

[rongon]

Good idea about adding duct seal around the KAB add-on weight and weighing the total. I guess I need to go buy a little digital scale.

I found an interesting DIY additional counterweight here --
DL103SA VS. SL1200: How I won the battle and lived happy (for a while) - The Art of Sound Forum. It's ugly, and it looks fiddly, but it looks like I could do it.

I found some specs and a price on the Jelco 750D here -- Jelco SA-750D 9" Tonearm | USA Hi-Fi. It looks like it still won't accept the 14.4g Soundsmith Gold without a little extra weight (stated maximum cartridge weight is 12g). I suppose 2g wouldn't be too difficult to put on there, somehow.

I think I might change my RIAA preamp's output stage to a cathode follower using a 6DJ8, instead of the common cathode 5687 it is now. That should give me a more 'standard' 40dB of total gain from the preamp. I didn't like it that the subjective level from my records was a couple of ticks lower than from the CD player, but I guess that's a small price to pay for being able to use the SUT in the system.

I have some work to do. Not today, though. Back to doing dishes!

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[vinylkid58]

I found some specs and a price on the Jelco 750D here -- Jelco SA-750D 9" Tonearm | USA Hi-Fi. It looks like it still won't accept the 14.4g Soundsmith Gold without a little extra weight (stated maximum cartridge weight is 12g). I suppose 2g wouldn't be too difficult to put on there, somehow.

You can get the 170g "heavyweight" c-weight for the 750D on ebay.

I think I might change my RIAA preamp's output stage to a cathode follower using a 6DJ8, instead of the common cathode 5687 it is now. That should give me a more 'standard' 40dB of total gain from the preamp. I didn't like it that the subjective level from my records was a couple of ticks lower than from the CD player, but I guess that's a small price to pay for being able to use the SUT in the system.

Is this your design or a published circuit?

jeff

 

[rongon]

Ah, then the Jelco 750D will definitely work. Good to know. It wasn't hard to find articles on how to fit that arm onto the SL1200. The arm looks well made, and looks good on the turntable, sort of like it belongs there. I like that.

My preamp is a variation on the Arthur Loesch circuit published in a Sound Practices article from many years ago. SY and others helped me get it right, in this thread, starting here: http://www.diyaudio.com/forums/tubes-valves/147717-cathode-bias-questions-3.html#post1886286. The final schematic happened way after that, which I've attached. I call it the "Gutbucket" because it's in a really ugly steel chassis. I took a lot of suggestions from that thread, wound up with LED bias for the first two stages.

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[vinylkid58]

Ah, then the Jelco 750D will definitely work. Good to know. It wasn't hard to find articles on how to fit that arm onto the SL1200. The arm looks well made, and looks good on the turntable, sort of like it belongs there. I like that.

Many choose the 750D because it works so well with the low compliance DL-103/103R.

My preamp is a variation on the Arthur Loesch circuit published in a Sound Practices article from many years ago.

Interesting, thanks.

jeff

 

[rongon]

I might have an easier way to get the gain lower in the preamp. I have a stash of 6J9P tubes, which I could use in the first stage instead of the 6J52P that's in there now. That should cut the gain in half, and let me use the transformers.

I think I can put together some kind of additional weight for the counterweight end of the arm. Maybe a bolt with the same threads as the KAB extra-weight, and some nuts to thread onto it.

Will take some experimenting, but should work...

 

[vinylkid58]

I think I can put together some kind of additional weight for the counterweight end of the arm. Maybe a bolt with the same threads as the KAB extra-weight, and some nuts to thread onto it.

It doesn't need to be pretty, it just has to work.

jeff