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My opamp based preamp, dual gain stage with pot in the middle. Works with many opamps

Luxman Amplifiers India

doors666

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I have been working on this preamp for a while now, it has a dual gain stage, the potentiometer sits in the middle of the two gain stages and is a 10k pot. The second stage has a switch, which can be used to turn the gain on of off. When the switch is in the off position, the switch is out of the circuit and the second gain stage acts as a buffer.

The default gain is 1.5x for the first stage and unity gain for the second stage (gain switch =off). For low voltage sources like portable devices, the second stage has a gain of about 4.2x, giving a total gain of about 6x. You can configure it as a buffer-pot-buffer also by skipping a resistor in the first gain stage.

There are some SMD 1206 components in this.

I am using dual 16.5v supply with this. Depending upon the opamp used and your output voltage requirements, you can also power it with 2 or 4 9v batteries.

The two channels are completely independent. They only share the pcb, you can use these for monoblock operation also.

I have omitted both input and output caps from the build as my amp has a 2.2uf input cap.

I tested it with quite a few opamps, with both using the scope and music. I checked the waveforms at 20hz, 1khz, 20khz. The load was 10k resistor and 3300pf capacitor. There are no oscillations with 200Mhz bandwidth setting of the scope.

I tested with the following opamps: LM6172, LME49725, NE5532, OPA1642, OPA2134, OPA2107, OPA2227, AD8599. Yeah, even the dreaded LM6172 is absolutely stable, in my opinion, it probably sounded best of the lot.

I have a few more opamps that I have that I wish to test with, these require a lower supply (max 15v), so I would modify the supply and test these out and post here. I also might have a few good ones in existing equipment, will check and test if some opamp looks worth it.

I tested a lot with familiar music, familiar speakers. These are a bunch of very good opamps and there are only minor differences. You can try with any of these with this PCB and you wont be disappointed. Its going to be more of which one suits your current system more.

I initially used panasonic fc 470/25v caps for onboard supply. These sounded fine, later I changed these to elna silmic 47uf/35v. There was a significant change, sound cleared up a lot. These caps do require some burn in time. Initially these sounded a little harsh and bright, but only after a few hours it improved a lot. I also want to try a panny FM here.

I did a lot of testing with single channel (the smd ones, I ran out of opamp adaptors), so the sound stage related observations arent there. I used foobar plugin for stereo to mono for this. Once I get more adaptors and find time, will post more info about stereo tests.

Here is the Link with detailed documentation like build guide, schematic, images, BOM etc. Click on the additional information tab once you goto the page.

The name is of course blues based, Blues Before Sunrise.:)

PIcs:





Schematic
 

omishra

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Re: My opamp based preamp, dual gain stage with pot in the middle. Works with many op

Nice one with lot of thought
 

doors666

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Re: My opamp based preamp, dual gain stage with pot in the middle. Works with many op

Some simulation results

Its doing about 2.95v into 10k Load with 2nf capacitance at full volume level, using NE5532:



2nd harmonic is about -115db, third harmonic is about -158db down, noise floor is below 180db.

THD is 0.000089% (Yes, thats 4 zeroes after decimal!!:eek:hyeah:)
 
Last edited:

jls001

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Re: My opamp based preamp, dual gain stage with pot in the middle. Works with many op

Wow!
That's an unbelievably low noise floor.
Do post actual measurements if possible, and your listening impressions. Thanks.
 

greenhorn

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Re: My opamp based preamp, dual gain stage with pot in the middle. Works with many op

EDIT: just saw its a simulation.
 

Sumanta

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Re: My opamp based preamp, dual gain stage with pot in the middle. Works with many op

Bravo!
I want to learn these things.
 

doors666

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Re: My opamp based preamp, dual gain stage with pot in the middle. Works with many op

After shifting it into my music system, I havent tried any opamp rolling, its been running on the 5532s.

Sound quality is simply amazing, such a cheap system has no business sounding so good. The only other decent preamp I have is the group buy B1 with the LDR and r-core transformer, in a nice split chassis for preamp and supply. BBS peacefully won against the B1. Sound is cleaner, crisper than B1. My wife could tell from the other room that I changed something and it was better. Some of my musician friends also preferred this over B1. ymmv.

I also have another pcb coming soon with a dc coupled jfet follower with ccs (with smd pads also for other other jfets like bf862) and a shunt regulated supply on a single pcb. This one I made to be a compact desgn for fitting inside power amps to make them integrateds. I am also working on a clean tube buffer using ecc88, Lets see what happens in comparisons then.:)
 

spirovious

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Re: My opamp based preamp, dual gain stage with pot in the middle. Works with many op

I prefer dcb1 over b1.l feel there are many things which could be missed in b1 easily.
Congrats for your new preamp.
 

saikatbiswas82

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Re: My opamp based preamp, dual gain stage with pot in the middle. Works with many op

Congratulations! Nice build and thanks for the detailed writeup.

Sent from my Nexus 5 using Tapatalk
 

kaushik

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Re: My opamp based preamp, dual gain stage with pot in the middle. Works with many op

easy front panel

1.3mm acrylic
2.make panel text in image app , go printo (printing store), get printed to vinylsticker
this lasts years also waterproof
 

doors666

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Re: My opamp based preamp, dual gain stage with pot in the middle. Works with many op

I have kits available for this, all the documentation, images, pricing etc can be found here.

Parts details:
The electrolytics are nichicon muse kz 33uf/25v.
The SMD caps are AVX skycaps.
All resisors are 1%.
Opamp is NE5532. This is a good opamp that performs quite well. Opamp rollers can use any opamp mentioned earlier.

Switch shown is for switching Gain.

I have tested it with the LM317/LM337 supply as well as the shunt regulated supply. Kits for both are also available on the site.

Pot is not included as people usually like to use their own. It uses a 10K dual log pot which can be procured from audiocraft. If a lot of people are interested, I can get alps 10k from RS also.

Price is 1500 as of now, shipping is 100.
 
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