TC-750 Phono Pre-Amp (Continued)

Well a follow-up to the post above...

I removed the caps that filter the power, and removed/jumped the diode, and the 1K input resistors, and ended up with a pretty noisy preamp.

But bass seemed improved.

I've been trying a few different wall-wart type power supplies, both switching and units I believe to be unregulated (just transformers and a rectifier circuit). They all resulted in about the same amount of noise.

I reinstalled the diode imagining that it was serving to block some RFI or AC on the line, and it is back to being fairly quiet, but haven't had a chance to listen any music.

My unit is the TC-750LC with PCB v3.

BTW when testing it on the bench, I'm using some ear buds on the output and various RCA cables on the input. A long (25') RCA cable apparently provided enough capacitance that the unit is actually very quiet.

So I wonder, I've removed the input caps on this thing, many mentioning the capacitance in the cables being sufficient. Does anyone know how much capacitance is sort of built into the circuit itself?

My turntable cabling including arm wiring and RCA cable is very low, around 83pf according to my measurements (I have a good meter). I figured the circuit itself may be providing 50pf or so even with the loading caps removed, but I have no idea.

Anyways, that is where things stand.

I guess my next thing to try, is various ceramic caps for loading to see what it takes to make it fairly quiet.

 

Hello
Just a couple questions, the consensus seems to be that the Sola SLS 12 V 3,4A Power supply is the one to go with?
And the input caps 3,9-4uF , output caps 10uF is the perfect balance for this pre?
Take the input 1K resistor out?
thanks,

 

Just a follow-up to my Jun 25 post. I circled back to this, hoping to get the noise down to at least match my Realistic 42-2109. I had removed the two 1k resistors next to the 200pf input capacitors, following the post made by Pyruvate (I have the same board).

I found these needed to go back in. Reinstalling them instantly dropped the noise substantially.

I compared the response to my upgraded (via kit I got on eBay) Realistic 42-2109. I need to use some loading plugs to tame the high frequencies, and I'm waiting on a 24v power supply which I'm told will pickup about 1.25db in the bass.

 

Hello,
The schematic posted earlier lists 2SC1815 transistors, my unit has STC945 in, is there a difference?

 

Hello,
What's the benefit of the R16 at 470 ohm, looks like a dropping resistor?
Should that one go out as well? Just to get more volts?

 

Another update...

My power supply of choice has been a 24v linear regulated wall wart. The TC-750 was very sensitive to hum based on the routing of the cord from the wall wart to the preamp. Hum was elevated regardless of best efforts, but could really be high if I didn't make an effort to route the cord "just right."

I had figured I should twist the wall wart cord to reduce its susceptibility to magnetic coupling to AC power cords. Asked around, the consensus was to reinstall the filter caps I had removed from the board.

But the caps I removed were only 25v, and I'm using a 24v power supply. Most thought it fine, but I scrounged through a cap assortment I had and found two 470uf 50v caps and installed those, and reinstalled the 470-Ohm resistor.

The hum and some RFI I was getting really dropped now. Very, very quiet. As quiet as a MoFi StudioPhono I previously had.

So in a nutshell my changes are: (1) 24vdc linear regulated wall wart. (2) 470uf 50v caps to replace the 1000uf 25v caps, to make sure the increased voltage is okay. I'll likely replaced these with 680uf 35v, as they'd be the largest I can comfortably fit on my model. (3) Pull the 220uf input capacitors. (4) Replace the input and output caps, I used Elna Silmic II caps, which are still electrolytics but they sound very good to me. (5) I've replaced the ground wire with a 6" wire with a crimped ring that goes under the left PCB mounting screw. In this way, I can remove/reinstall the board easily, without having to deal with any wires needing to be desoldered.

As I've progressed, I've continued to record samples from various test records. My total cable capacitance is around 83pf, no additional capacitance on the TC-750, and I get a very nearly flat response curve out to 20k. This is exactly what I was after.

I had planned on installing and leaving sockets so I could swap the loading resistors, but find 47k is really fine. My response curve is flat enough that any minor deviation I might get from slightly higher or lower resistance would be down to the accuracy of the test records. So I've soldered the 47k resistors back in.

 

With new M caps, mid price decent capacitors . The unit sounds very good, but it has a hum at high volume, I will try to replace the filtering caps as cgallery did.
thanks

After inserting the 1000uF filtering caps back in, I can say it did not make a difference in the hum, but the case is open due to the new large caps, so that might have something to do with it.( the 2 1000uf filtering caps will stay in, extra RC does not hurt.)
I also used 82pf loading caps instead of the 220pf factory supplied.

Anyway, the unit sounds better than before the 2 sets of coupling cap change.
Also, Breaking the mundorfs MCaps in will take a while.

 

I've been switching around phono preamps quite a bit lately, and have had a few chances to come back to this one for some critical listening.

My power supply preference is the 13.8vdc linear regulated (inexpensive Pyramid brand) used to drive my turntable motor. Very low noise (still have to be mindful of how the power and signal cabling is run) and the music is delightful, great bass and sparkly yet delicate highs.

The 12vdc switching wall wart that came with the TC-750 comes in 2nd place. A little more noise, maybe the bass isn't quite as authoritative, but not bad, not at all.

The 24vdc linear regulated comes in a very distant third. Running with the 24vdc power supply causes a pronounced emphasis in the bass (up to 1db @ 40Hz), and the entire sound is distorted, like you're listening through a compressed FM radio signal.

I'd suggest staying close to 12- to 15vdc.

So basically I only run with two modifications: Replacing the stock 220pf loading caps with 100pf, and using the 13.8vdc linear regulated power supply.

So I guess my advice is, if you make a bunch of changes and get a lot of noise or think it doesn't sound great, put is all mostly back the way you found it.

Next up, I'm going to replace the mylar caps in the RIAA sections. This is where cap changes should really be audible, I think less so on coupling where the reactive frequency is so low as to be less audible. And mylar is perhaps one of the worst choices for RIAA, from what I've read.

 

You guys have 2SC1815 and STC945. Those seem equivalent while mine are actually H945 and slightly different: https://www.npntransistors.com/c945-transistor/

• The equivalent transistor for C945 is 2SC1815, 2SC2458, 2SC3198, C1815, and KSC1815.

I didn't get a switcher/cord/plug with my TC-750. Does anyone have a part # for the barrel plug?

 

The connector is a 3.5x1.3, I think a 3.5x1.35 is essentially the same thing.

I've used these:

https://www.amazon.com/Conwork-10-Pack-Adapter-Straight-Connector/dp/B01G6EB99E

...and they work fine. I did notice that unlike some other DC plugs, it is possible to push a paperclip all the way in and have it short the inner and outer conductors. I sometimes use a paperclip to measure voltage of these DC plugs and I don't think I've ever had a paperclip pushed into the center be cable to insert beyond the end of the plug and into the solder terminals, so this was a bit of a surprise. Not a problem as long as you're mindful if using a paperclip to help measure the voltage.

 

^Thank you @cgallery

 

Hello,
I Have Read somewhere when replacing electrolytic caps with film caps in signal path you don't have to use 10uF and 4uF values such as in this unit, ( this is due to inferiority of Electrolitycs,) in fact 2uF and 1uF in film caps should be sufficient, that would make it easier fit in the box....
anyone would know if this is correct?

 

The capacitance will dictate the frequency of reactance, too high and you'll sacrifice bass. So I'd say that tip is false.

 

Hello,
I had put in bipolar Mundorf Raw capacitors 10uf and 4,7uf and still was not happy with the results.
Fast forward a couple of months I ordered poly caps to replace all of the cheap caps in the RIAA stage and also put in 4uf input and 1,5uf output polypropylene film caps and the results are stunning. OC bearing in mind it’s still a small RIAA preamp, but now it’s very capable …. And worthy of using as a HIFI pre.

 

cgallery has asked me a number of questions about the 750 by PMing me on Audiokarma. I have created an LTspice schematic and I can easily simulate noise, frequency response, RIAA compliance, effect of component changes, transistor changes etc. etc. If anyone needs me to look at anything I am willing to do so as it's generally a matter of moments, and he has found it, I believe, quite useful.
https://www.dropbox.com/s/jpvh91554ez5r6n/750.jpg?dl=0
By the way if anyone knows how I can post an image of the schematic I would appreciate it.

 

Wyn, they apparently don't allow attachments at this site, photos have to be hosted elsewhere.

I opened the link you posted, then did a right click on the image and selected "open image in new tab" (I'm using Firefox BTW). This opened a full-size copy of the schematic. I copied the URL of the new tab and pasted it into the box that appeared when I click on the image icon in the message editor here.

But I don't know how long Dropbox will keep this at the same URL. I've saved the image on my machine and will re-post it if it vanishes.

Thank you again for all your help, people with your level of education and experience that are so generous with their time are very few and far between!

For those that don't know, Mr. Palmer is a senior fellow @ Analog Devices (I believe recently retired), and has designed his own phono stages for those that are DIY inclined (more info @ AudoKarma.org, in the Turntable forum), as well as the new very-well-received phono stages being manufactured by SOTA (who apparently struggle to keep up with demand).

 

Thank You!

 

Well alright, I'll ask a question.

The specs on the webpage:
https://www.phonopreamps.com/tc750pp.html

...indicate a nominal and maximum output of 300mv and 1.8v, respectfully. 1.8v seems sort of low, given that the manufacturer has also told me they routinely use cartridges rated for as much as 10mv output (@5cm/sec).

Some people are apparently running these at much higher than the "rated" (12v) voltage. I've read (here in this thread) 24v is too high (the unit can apparently sound strained). I run 13.8v (via a bench power supply).

My question is, does the model allow you to determine the maximum input and/or output voltage before clipping at 12v, 13.8v, and 18v?

 

Absolutely, however it will most likely not be one number per voltage.