Single chip MC phono stage

[DrewP]

Had a look recently on the links page for DIY audio.

On page 3 of amp projects there was a site Lidstrom Audio who had a schema for a single opamp MM stage.

Usual deal, 100pf and 47 k across non inverting pin.
The inverting using 10R to ground with a 10k feedback loop bypased by 0.33uf in series with a parallel network of 0.1uf and 787R.

Was wondering....

If you only plan on using an MC (Ortofon MC 20 Supreme for instance with about .7mV output)...

Is it an option to drop the input load resistor down to 100R, dispense with the input cap totally and change the 10R to ground to a 2R to boost the gain? Was contemplating use of AD797.

Am I missing some really obvious detail here or is this a workable solution?

Most MC designs seem to be a tack-on solution to match a 100R to a 47K with the extra 10x gain.

If a single chip mm stage can work, would a single chip MC stage be impossible?

I expect that noise and distortion across the really high gain stage will be the killers for this, just seeking some opinions.

In anticipation...

drew

 

[jackinnj]

Search these pages on AD797

There are pro's and con's of using this chip -- it is bit expensive, but since your labor is more expensive, I guess that's a wash. I was thinking to build the multiple transistor pre-preamp and replace the unit I now have.

I have used the AD797's in an RIAA amplifier, the sound is good but, quite frankly, I think that the surface noise on my LP's mitigates any benefit of using such an expensive chip.

 

[Jocko Homo]

More obscure AD RIAA circuit

The preliminary data sheet for the AD846 showed a circuit for using it in one. They dropped in in the final version. I may still have a copy here somewhere. Talk about expensive.......

Jocko

 

[Jocko Homo]

Found it!

Here it is, zipped up so it will fit here:

Jocko

 

[dice45]

Folks,
search after posts from concerning a single-FET MC headamp. I remeber i posted details.
Could be this thingie outperforms any chip solution and competes with the best MC-headamps and trannies avaialble. It uses a 2SK43 and runs from a pack of NiCd batteries.
it is in fact a voltage-controlled voltage divider with a gain of 10.

 

[DrewP]

I wasn't seeking a headamp stage in particular. As folks have noted both add-on headamps and opamp based phono stages are relatively common.

What I was more particularly curious about was the possibility, practicality or problems of using a single opamp as the whole phono stage (mc input, 1000x gain and eq all together).

Many thanks for all other input, I'll read further on the more traditional approaches as well.

Drew

 

[DrewP]

Done some looking folks, and without going valve and getting relatively scary, there's not a lot of simple phono stage schematic out there.

This brings me back to the original question:

In december of 1993 in the 6th HiFi World supplement, there was part one of an article published on using an AD797 (preferrably but other options were given) as a single stage mc amp with cartridge load setting resistor from the + terminal to gnd and gain setting resistor from the - terminal to ground with the riaa network sittng in the feedback loop.

Unfortunately, for reasons of long cable driveability etc, by the time section 2 of the article appeared some months later (in which details of the riaa network were to appear) the design had been given a second opamp on the output and a passive 'shunt to ground' riaa network between opamp1 and opamp2.

Given that I don't wish to drive long cables (the phono stage will be very local to my preamp) I like both the simplicity and the elegance (if folks can come at using an opamp as ever being elegant) of the single stage circuit.

So, can anyone see how the Lidstom cct listed in "links - amplifiers - projects" would malfunction if the input res was set to 200 instead of 47k and the gain setting resistor was dropped to 2 ohms from the 10 ohms it currently is?

FWIW, I'll likely build the prototype using an NE5534 just to check behaviour and see how things go.

Thanks in advance

Drew

 

[DrewP]

It Works!

Although I have yet to hear it, electrically the cct I constructed operates fine.

I've constructed the cct shown on the Lidstron page but modified things a little. The 10k resistor incorporated into the feeback loop has been dropped to 8k2 (think the original was setup to apply a little more bass boost than strictly accurate). The gain has been boosted by dropping the value of the 10R gainsetting resistor down to 1R5 and the input resistor has been changed from 47k down to 100R. As intended for the trial, the opamp used was NE5534AN. Caps were bog standard MKT and resistors were 1/2 watt metal films of pleb parentage. PSU is two 9v cells.

Tests to date give the DC offset at output around 3 volts so I'll install an output cap.

0.7mv in gives a nice 550mv out. Clipping doesn't set in till around a 3mv signal is fed into the input.

Response to sqaure wave from 20 - 20k is within about 1dB.

Reports of sound quality to come as soon as available.

Drew

 

[peranders]

Had a look recently on the links page for DIY audio.

On page 3 of amp projects there was a site Lidstrom Audio who had a schema for a single opamp MM stage.

Usual deal, 100pf and 47 k across non inverting pin.
The inverting using 10R to ground with a 10k feedback loop bypased by 0.33uf in series with a parallel network of 0.1uf and 787R.

Was wondering....

If you only plan on using an MC (Ortofon MC 20 Supreme for instance with about .7mV output)...

If a single chip mm stage can work, would a single chip MC stage be impossible?

drew

Check my QSXM3 amp which is more or less a blueprint of the Linear Tech datasheet of LT1115.
http://www.linear.com/prod/datasheet.html?datasheet=202

This opamp can be used as a single chip solution for MC pickups.

 

[transducer]

Yo Jocko!

What's the value of that cap under the "M" in "preliminary"? It looks like 0.47pF?

If I wanted to do this as a standalone unit, replace the 10k pot with a small value resistor, like 75 ohms? Haven't checked the spec's of the 797 vs. the 846, but I have some of those 797's lying around, think they'd work?

Cheers,
RonS

p.s. got your jokes, was away in the bush

 

[Jocko Homo]

The cap is 0.47 uF, as it is a second order servo. One of Harry's least favorite audio nerds says that is a no-no.

Replace the 10K with 75 ohms? I assume you mean for series back termination.

I've used the '846 for I/V stages. It sounded good, but no one liked. Hard to explain why. You'll just have to trust me. '797s sound good. Jeff Rowland likes them.......

Jocko

 

[transducer]

Jocko,

So, servo's are no good, or second order servo's??

I like how Kevin Gilmore did his offset nulling on his discrete high current headphone amp, non invasive.

Thanks for the cap value, I'll try with 797's one of these days, no moola to buy new parts so no 846, no work you know

Later,
RonS

 

[transducer]

Jocko,

Forgot, yes 75 ohms for series back termination to prevent oscillation. I once built a line stage based on the 797 with no series resistor, my amp got so hot I could fry an egg on it before I realised what had happened Never made that mistake again.

RonS