Anyone tried DIY'ing cheap, clean mic-preamps?

[PRHunt]

Been looking at Self again and the AD797 is about the lowest noise IC around being some 10dB quieter than the already very good NE5534. It costs 10 times as much however being £8.18 +vat here from RS.

If that noise improvement can be realized in a practical circuit it would beat the best of the hybrid transistor/IC designs it seems? I might jeeeeust push the boat out for my birdy amps!

The THAT chips are very clever and there is at least one commercial pre amp on the market using them but their main claim to fame is I understand their very high CMRR? Not of great interest to the home studio bod unless he is plagued with very high interfering fields.

Oh! The wife Paul! Thanks, yes she was very much brighter today, seems she is on the mend after some very tough abdominal surgery.
Dave.

It is interesting that the AD797 and the NE5534 are frequently targeted for "upgrading" with newer and sexier chips, when in reality, the performance of these older chips is pretty blameless. There is one high-profile modder (Jim Williams, Audio Upgrades) who strongly advocates the use of high slew rate video op-amps for audio. I'm not convinced that this is really necessary.

If I have read correctly, the THAT 1512 is a monolithic implementation of the Cohen topology. Apart from high CMRR is low, they also specify low noise at high gain. I believe that these chips are used in the current iteration of the DAV BG-1.

I wish your wife a speedy recovery. Abdominal surgery means that... mmm... other family members will need to step up for household duties. For six weeks at least...

Paul

 

[ecc83]

It is interesting that the AD797 and the NE5534 are frequently targeted for "upgrading" with newer and sexier chips, when in reality, the performance of these older chips is pretty blameless. There is one high-profile modder (Jim Williams, Audio Upgrades) who strongly advocates the use of high slew rate video op-amps for audio. I'm not convinced that this is really necessary.

If I have read correctly, the THAT 1512 is a monolithic implementation of the Cohen topology. Apart from high CMRR is low, they also specify low noise at high gain. I believe that these chips are used in the current iteration of the DAV BG-1.

I wish your wife a speedy recovery. Abdominal surgery means that... mmm... other family members will need to step up for household duties. For six weeks at least...

Paul

Ah! the "Beatifiers"!! Nutters all IMHO. The only op amp ever to enter the audio market with an indequate* slew rate was the 741, all others have slew in spades. To produce a clean 20khz sine at say~10V rms you need a slew rate of a smell over 2V/muSec. The 5532 is about 4x better than that and the TL072 about 6!

*The 741 at 0.5V/muSec was really quite adequate for the signals of the day, its downfall was noise.

In any case I live by the maxim (P J Baxendall I think?) "The wider you open the window the more the muck flies in". Thus I want my kit to have a cutoff of about 50kHz and if that means it is 0.1dB down at 20kHz? Well, I can live with that. I have never HEARD 20kHz in my entire life!

Homebod? Yes Paul. She has been pretty poorly for quite some time and so I have been doing more and more (I was always head cook but she was an ace washer up!) . She had a completely ***t GP for 50yrs. Daughter and I have finally got her to move to my surgery.

This means I have had to give up my part time job with the gitamp peeps* and I shall miss the several 100 a month! But, we managed before. We shall again.

*Mind you that might have happened anyway. Best perhaps I go this way.

Dave.

 

[JG96]

I have a Shure SCM-810 that I use for its Pre's. Its 8 channels and they go for under 100 on ebay.

 

[ecc83]

I have a Shure SCM-810 that I use for its Pre's. Its 8 channels and they go for under 100 on ebay.

That is an interesting beast. It is an "automatic" mixer which means it keeps a set output level for varying mic input levels but avoids feedback.

The development of the idea goes back to the '70s and I have an AES journal somewhere with the original paper. It uses "Adaptive Logic Gates" IIRRC but I did not know that there was still a commercial version around?

I have found the user manual for it and it employs the strange value of 46V for phantom power. That is just at the bottom of the standard specification for spook juice but most, mics will not mind (tho a very few just might!) .

Good basis for a DIY project I would say!

Dave.

 

[ecc83]

New info and further thoughts....
The current issue of Sound on Sound has a review of the Focusrite Red 1 500 pre amp.

This uses a Lundahl 1538 input transformer with a 1:5 ratio into an NE5534 and delivers a noise figure of some -130dBu. That is getting close to theoretical limits.

Those with ribbons or SM7bs might be interested? (in a build that is, the pres are 800quid. Each!)

Now, I am NOT going to spring for Lundies for the Wife's bloody birds! But I shall have a look at what OEP can offer in that ratio.

Dave.

 

[ecc83]

Just found 3 unused THAT 1510P chips. Free to a good home just PM me with an address and I shall jiffy them to thee!

Dave.

 

[drtechno]

been working on a grounded grid DC coupled transformer less tube mic pre (60% complete). but when I'm finished, I will start designing an op amp based.

I already have a pretty good idea on the op amp based- a transformer less version optimized for sm7/sm57 applications.

BTW EE was my major, music was my minor in school.

 

[drtechno]

5534 is a good op amp. I use them like SSL does at +/- 20V in there consoles.

the AD offerings are great too. and there is something about it when you hook up a 12k-20K resistor from there output to the positive rail --- they impart an old '70 console warmth. It seems to operate more in the class A region.

 

[ecc83]

5534 is a good op amp. I use them like SSL does at +/- 20V in there consoles.

the AD offerings are great too. and there is something about it when you hook up a 12k-20K resistor from there output to the positive rail --- they impart an old '70 console warmth. It seems to operate more in the class A region.

Dunno about "class A"? The practice most likely produces more even harmonic distortion, still, whatever floats yer boat!

I I would not run NEs at anything like 20+20 volts. Yes the absolute maximum spec is 22+22 but at 2x 17V you can get +22dBu before clipping and an easy +26dBu from a balanced pair. Surely this is enough for any A-D converter? The chip will run cool and near 100% zero failures.

Dave.