Vintage Mic Transformer for MC step-up - gain-phase and impedance measurements

[Gold]

You could "divide and conquer" by splitting up the issue, like driving the phono input from a signal generator, thru the transformer, or two of them back to back if generator does not have 5 ohm output ;-).

You’d think that might work but it’s only good for less tweaky measurements. The only reliable way I’ve found to tweak out a phono cart is mounted on a turntable playing a record. Maybe because the generator doesn’t mimic the L and C of the cartridge coils?

Mr. CMRR described complex circuit interactions and that’s what I hear. It’s hard to put your finger on exactly what’s happening.

The KA Electronics PTS can be constructed in many ways. My request for the design was a remote mounted preamp with a balanced front end. For playback from the lathe.

In this day and age I can’t understand why the standard phono interface isn’t balanced. It would improve playback sound quality more than the best of everything would. For both MM and MC carts.

I haven’t built Wayne’s MC front end but it’s on the list. I plan on using the MM front end with a transformer in front of it.

That’s what I’m doing now for the DL103 on the lathe and what I will do for the SPU on the Shaker Phono System I am in the middle of building.

It will have three tonearms with a Denon DP80 turntable. Each arm will have a dedicated PTS and each cartridge will have a dedicated front end.

 

[Cqwet Dbdfte]

Not sure how much, and what a differential output from a cart would help.
One of the 4 wires are typically attached to the case, or some shield, which is then connected to a shield of a coax cable.
I guess if you remove this connection you could add a pseudo ground voltage divider and feed a diff amp.
With long wire feeds this may be good, have not tried. Shielding from external electrostatic and magnetic fields is a high priority, where the tone arm does the job to some extent.
60Hz fields are best reduced with distance. Magnetic shielding is not easy nor cheap.
The transformer should break up ground loops, but as I found out can be an intrepid magnetic field detector.
Toroidal power transformers help, and distance adds to to this.
Changing the cart wiring standard may be too late.

 

[Gold]

Not sure how much, and what a differential output from a cart would help.
One of the 4 wires are typically attached to the case, or some shield, which is then connected to a shield of a coax cable.

Want switch over those SM57’s to an unbalanced interface? It makes a massive difference. The difference between a gaussian noise floor and a “tizzy” noise floor. I can crank my monitors to stun and all I hear is hiss.

Sometimes there is a ground strap on a cartridge. Sometimes like on a 1200 the strap is internal. There are some issues but nothing hard to change.

 

[Cqwet Dbdfte]

Chasing low level EMI is not a fun task, EMC regulations address this problem, but a lot of stuff barely meets when new, and age does not do equipment any favors.
Power supplies should be super clean, some old Fluke meters has NiCd batteries for offline measurements, pointing to the source of problems.
Rectifier diodes generate noise that can be both conducted and radiated out. Changing to lower noise types should be a worthwhile enterprise.

 

[Gold]

Chasing low level EMI is not a fun task, EMC regulations address this problem, but a lot of stuff barely meets when new, and age does not do equipment any favors.
Power supplies should be super clean, some old Fluke meters has NiCd batteries for offline measurements, pointing to the source of problems.
Rectifier diodes generate noise that can be both conducted and radiated out. Changing to lower noise types should be a worthwhile enterprise.

The main source of noise in any unbalanced phono system is induced noise in the cabling. None of the stuff above helps that. Which brings up a giant pet peeve of mine. Phono preamp designers obsess over a fraction of a dB of EIN while that noise is almost always swamped by interface noise. When they slap a resistor across the input and quote noise figures it is damn close to lying because you will never see those figures in a real world setup.

Of course a balanced interface makes all this basically moot.

 

[Cqwet Dbdfte]

The biggest noise source in the phono system is the needle sliding over vinyl...
In my case, like many, cabling is very short, like 3', I cannot hear anything needle up unless I put my ear into the speaker, which is getting 1mV of noise from tube amps, with AC filaments.

You could place the signal transformers inside, or close to the turntable, away from motor windings.

 

[Gold]

I’ve been dealing professionally with lathes and phono systems for over 25 years. I can get a turntable next to a receiver in the back country very quiet.
My monitor level is generally about 93dB spl. You might not have to put your ear up to the speaker to hear the crud at that level. I listen to lacquers mostly. The noise floor is better than 30 ips tape. Better than -65dBu. The PTS is round about -70dBu depending on cartridge. It’s a gaussian noise floor.

 

[Gold]

The best I've ever been able to do with an unbalanced interface in NYC is in the neighborhood of -45dBu-55dBu with a bunch of crud in the noise floor. That's with about 1.5ft of cable between the tonearm and phono pre. What you got?

 

[abbey road d enfer]

The best I've ever been able to do with an unbalanced interface in NYC is in the neighborhood of -45dBu-55dBu with a bunch of crud in the noise floor. That's with about 1.5ft of cable between the tonearm and phono pre. What you got?

That's a quite low figure. Is it with MC or MM cartridge?
I just have a basic AT turntable and a Technics preamp and the electronic noise floor is way lower than surface noise of many discs.
With proper electrostatic shielding, an unbalanced connection can achieve excellent S/N ratio, since there is no ground loop. Now there may be significant EMI/RFI...

 

[Gold]

That's a quite low figure. Is it with MC or MM cartridge?

Yes it is With both the MC DL-103 and an MM Shure V15 Type V

I just have a basic AT turntable and a Technics preamp and the electronic noise floor is way lower than surface noise of many discs.

Remember I'm checking lacquers. The noise floor can be better than -65dBu.

With proper electrostatic shielding, an unbalanced connection can achieve excellent S/N ratio, since there is no ground loop. Now there may be significant EMI/RFI...

The RF/EMI is the problem. Even with short cables the best I can do is a dirty -55dBu noise floor.

 

[Cqwet Dbdfte]

Gold, at what point do you measure the noise?

 

[Gold]

Gold, at what point do you measure the noise?

Cartridge hanging in free air on the tonearm. Measure noise at output. Calibration is 1k lateral at 7cm/sec = +4dBu.

 

[Cqwet Dbdfte]

Output of....?

 

[Gold]

Output of....?

Your reproduction system. Output of your phono preamp? Output of your line stage? Take your pick. Wherever is convenient. Got a test record to calibrate?

 

[Cqwet Dbdfte]

Setting up my ancient dB meter for 0dBu, (2.2V p-p on the scope) full scale, at with a 1KHz signal, and measuring after the (unattenuated) phono RIAA preamp with 48dB gain, I'm measuring -55dBu on one channel, and -45dBu on the other.
The phono preamp is fed by a 22.5:1 transformer (27dB), and has two gain stages, with a passive RIAA filter in the middle. The gain is done with two tubes, a 7721/D3a and a 6DJ8, both plate loaded with constant current sources of 20mA and 10mA respectively. I'm guessing one of the current sources is 10 dB quieter than the other. The amplifier gain at 1KHz is about 48dB, not including the transformer.
I measured with the turntable motor, and Line Amp running.
The (tube) line stage is basically just an attenuator, but has about 10dB gain at max out. I'm debating moving my phono stage into the Line Box, which has a convenient remote control for a relay stepped attenuator, as its extra 10dB is not really needed.
The phono stage has massive output range, like 155V peak to peak, and can drive a 600Ohm load with a little loss. No negative feedback is used.

 

[Gold]

Without a test record all
Measurements are kinda meaningless.

But now terminate the phono input with a resistor the value of the cartridge and see what happens to those numbers.

 

[Cqwet Dbdfte]

A test record would add surface noise, unrelated to noise of the electronics. I'll try the termination resistor.

 

[Gold]

A test record would add surface noise, unrelated to noise of the electronics. I'll try the termination resistor.

Not measure noise of the test record. Calibrate to a Standard Reference Level. So the gain of the amplifiers are matched and the numbers actually mean something.

If you need remedial help consult the Audio Cyclopedia.

 

[Brian Roth]

Not measure noise of the test record. Calibrate to a Standard Reference Level. So the gain of the amplifiers are matched and the numbers actually mean something.

If you need remedial help consult the Audio Cyclopedia.

Yep!! Just like measuring the noise floor of a tape reproducing machine. You use a MRL alignment tape to establish an operating level, so you can then measure and state something like::

Noise floor is xxx dB below 250 NanoWebers per meter.

Edit: you do NOT run any tape for that measurement; all you are measuring is the noise floor of the reproduce amplifier.

I'm skipping over A or C weighting, etc etc.

Bri

 

[Gold]

Noise floor is xxx dB below 250 NanoWebers per meter.

The unit of measure in disk recording is velocity measured in cm/sec.

The standard professional calibration in the US is 1k lateral at 7 cm/sec = +4dBu = 0VU. Professional calibration records have 1K lateral at 7 cm/sec.