Help me please!

[Leek]

What if I were to use Vishay 6.8k 1% res at R1&R2, change R3&4 to Vishay 100 ohm 1%, R5 to 10k linear pot, R6 to Vishay 10k 1% and R7 to Vishay 3.3k 1%? Smaller resistors would decrease noise, correct?

 

[abbey road d enfer]

Understandable. Thanks for the information. If I built (http://www.tradeofic.com/Circuit/783-BALANCED_MICROPHONE_PREAMPLIFIER.html) using OPA2134's, how could I eliminate noise? When you say noise, are you referring hiss as in feedback like from a guitar amp? I've heard of the 5$ Mic pre, it sounds clean. How can I not achieve this with a OPA2134?

In terms of noise OPA2134 is just marginally better than 5532. If you think the performance from the $5 Mic Pre is satisfactory, then build it, but it is far from being close in performance to the "cheap" mic pre in your 002.

 

[abbey road d enfer]

What if I were to use Vishay 6.8k 1% res at R1&R2, change R3&4 to Vishay 100 ohm 1%, R5 to 10k linear pot, R6 to Vishay 10k 1% and R7 to Vishay 3.3k 1%? Smaller resistors would decrease noise, correct?

Only marginally; then reducing R3 & R4 would reduce significantly the input impedance, which the microphone wouldn't be happy with. This topology, which is perfectly valid for line inputs, is utterly inadequate for a quality mic pre.

 

[Andy Peters]

Understandable. Thanks for the information. If I built (http://www.tradeofic.com/Circuit/783-BALANCED_MICROPHONE_PREAMPLIFIER.html) using OPA2134's, how could I eliminate noise? When you say noise, are you referring hiss as in feedback like from a guitar amp? I've heard of the 5$ Mic pre, it sounds clean. How can I not achieve this with a OPA2134?

I think what everyone is saying here is: Read the data sheet for the INA163 or the THAT 1510 and basically build what they tell you to build.

Use a DRV134 for your balanced line output.

Done.

It does not have to be any more complicated than that.

-a

 

[JohnRoberts]

The not yet mentioned fact is that microphones output very small voltages, that require lots of gain, often 1000x.  To realize a good signal to noise the mic preamp needs to have extremely low input noise. A decent general purpose op amp with microVolts of input noise may be fine for processing line level signals (volts), but will appear noisy relative to mic levels (mV) and mic input gains (thousands).

Just because people post  schematics on the WWW called mic preamps using general purpose op amps, does not mean they will be quiet enough for serious applications.

You are lucky, nowadays you can buy several off the shelf dedicated mic preamp ICs where the heavy lifting has been done for you. As Andy suggested just follow the typical manufacturer application notes.  THAT, TI, and others make decent standard preamp ICs.

Back in the bad old days we would have to use a transformer front end to step up the input voltage  high enough for an op amp to deliver decent S/N, or design a circuit using discrete low noise transistors.

Take the easy path...

JR 

 

[Michael Tibes]

Why not build the 5$ mic pre? It'll probably be way better than reinventing the wheel with less than optimal parts.

Michael

 

[Leek]

It's about learning. Why not take the time to figure out how different preamp circuits operate in terms of noise, sound and etc?

 

[ruffrecords]

It's about learning. Why not take the time to figure out how different preamp circuits operate in terms of noise, sound and etc?

There is no better way to learn than by direct experience . I highly recommend it. You could do worse than configure you OPA2134 so as to realise the $5 mic pre.

Cheers

Ian

 

[Leek]

So, i've built the circuit. Sounds decent. Noise isn't to crazy, but noticeable at max gain. Just ordered a OEP Z21808C and Carnhill VTB-1148. Hoping to use the Z21808C at mic input stage, OPA2134 preamp stage and VTB-1148 output stage. Any tips or ideas for such a build?

 

[ruffrecords]

So, i've built the circuit. Sounds decent. Noise isn't to crazy, but noticeable at max gain. Just ordered a OEP Z21808C and Carnhill VTB-1148. Hoping to use the Z21808C at mic input stage, OPA2134 preamp stage and VTB-1148 output stage. Any tips or ideas for such a build?

The VTB1148 is a gapped transformer really intended for single ended class A output stages. If you want a transformer balanced output you should look for an ungapped transformer like the  VTB2281. Alternatively, since you have four op amps in the OPA2134, you could make an electronically balanced output.

Similarly, the OP2134 is quite a low noise device and arranged as an instrumentation amplifier with low value gain resistors and no series resistors in the inputs, it should be capable aof very good noise performance and it wi; not need an input transformer. On th other hand, the OEP Z21808C should give you an extra 16dB of relatively noise free gain.

Cheers

Ian

 

[Leek]

Grabbed  2281's as well. Ian, by chance could you provide a simple OPA2134 for me to test?  My circuit (http://www.tradeofic.com/Circuit/783-BALANCED_MICROPHONE_PREAMPLIFIER.html) sounded ok, but was no where near the quality of my 002 pre's. I've also picked up a bunch of BC549's, 2N3904's and 2N4403's to dabble in a discrete class A input/output. Which is where the VTB-1148 idea came from. I planned to devise or at least test a Class A output stage with the 2N3904 transistors and VTB-1148.

 

[ruffrecords]

This one you posted earlier is the one I was refering to:

http://www.8085projects.info/image.asp?picture=Balanced%20input%20microphone%20amplifier%20using%20NE5532.jpg

Replace the NE5532s with the OPA2134. Change the 51K resistors to 5.1K and delete the 510 resistors at the input. Change R5 to a 5K pot with a 100 ohm resistor in series . Change the four 20K resistors around the third op amp to 4K7  and change C4 to 100uF.

Cheers

Ian

 

[Leek]

Thanks for all your help. I've just finished the circuit. Ran a SM57 through it (XLR Input to 1/4" Output) on back to line in on the 002. I used a splitter to test 002 pre vs mine. First design was a little undersatisfying. But, this revision is a lot better. 002 pre is very neutral and sounds good (maybe I should take a peak inside?) but the OPA2134 sounds just as good. Maybe it's just pride because I built it. But noise to me is inaudible and sounds really good compared to the 002 side by side.

Next up is a 4-Band sweepable EQ. (http://m.dextrel.net/site/mobile?dm_path=%2Fdiyequalizer.htm&fw_sig_permission_level=0&fw_sig_locale=en-US&fw_sig_partner=webs&fw_sig_url=http://www.dextrel.net/&fw_sig_permissions=none&fw_sig_site=32015199&fw_sig_time=1467763846809&fw_sig_api_key=522b0eedffc137c934fc7268582d53a1&fw_sig_access_token=970e6ec3a13f1eb0097ab9fb53ac67b4a2d85767&fw_sig_tier=1&fw_sig=275e37f6531a8fb021b3637957599aff&fw_sig_session_key=3244f1b4f3f4c568b5b87b6c69b973ac6f2976a6d02426ca90d188c0f53fc544-32015199&fw_sig_potential_abuse=1&fw_sig_premium=1&fw_sig_is_admin=0&fw_sig_social=1&fb_sig_network=fw#2010)

Though I'll be using better op amps. NE5532, OPA2134, OPA2604 or AD8512 to be exact. Once complete I'll send the out of the mic preamp into the input of the EQ circuit. I have a simple transistor based 1-knob compressor I'm blueprinting as well. Sigal path will go Mic preamp, compressor, EQ. I have an old empty Carvin 12 channel console frame I plan to retrofit everything in.

 

[buildafriend]

Go with the THAT chips if you want inexpensive but fairly high performance parts. I think there is some discussion on here about them and there may be some PCB's available if that tickles your fancy. I remember something like a feeler for them or some being sold.. it was a while ago. You might develop an appreciation for DOA's and good transformers though.  For my first design I went with the API 312 topology. 

http://www.thatcorp.com/1510-1512_Audio_Preamplifier_ICs.shtml

Good luck!