DIY Simple Active Preamp

ghazzer said:
Have you ever considered a step attenuator with 1% resistors? Not "cheep", but under $30 from Hong Kong. Maybe we could get Arkay to check them out.:D

Grainger 49 sent me this link for the S5 Attenuator, but I haven't had a chance to read & digest it.
.
Click to expand...

one word, Benjamin. TVC (well, more accurately, AVC).
 
Alps pot is OK, but real tovarishch will use CCCP mil. spec. stepped attenuator. Carry on Comrad, udaci!
 

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HypnoToad said:
Don't laugh I just threw it together to see if it worked.

DSCF1980.jpg
Click to expand...

:lmao:....:lmao:....:lmao:....:lmao: Now I know what to do with those tubs when the smoked turkey is all gone.

Yeah, me too.

:lurk:

Ok a few thoughts after reading all the posts.

HT, the schematic shows a dropping resistor before the pot. If you adjust the gain loop you don't need to shed any of the signal coming into the pot.

A good source has a low source impedance. The resistors after the coupling cap give it a high source impedance. Without the cap you shouldn't need a series resistor or a bleeder to ground.

I have read, haven't experimented with it, that high resistance pots as an input can create noise. So if this is true a 25k pot would be a good input impedance and, hopefully, lower noise.

I'm not sure about this, but, if your positive and negative supplies (batteries) are not perfectly balanced there could be a DC offset.
 
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Grainger49 said:
:lmao:....:lmao:....:lmao:....:lmao: Now I know what to do with those tubs when the smoked turkey is all gone.

Yeah, me too.

:lurk:

Ok a few thoughts after reading all the posts.

HT, the schematic shows a dropping resistor before the pot. If you adjust the gain loop you don't need to shed any of the signal coming into the pot.

A good source has a low source impedance. The resistors after the coupling cap give it a high source impedance. Without the cap you shouldn't need a series resistor or a bleeder to ground.

I have read, haven't experimented with it, that high resistance pots as an input can create noise. So if this is true a 25k pot would be a good input impedance and, hopefully, lower noise.

What I'm not sure about is if your positive and negative supplies (batteries) are not perfectly balanced will there be a DC offset?
Click to expand...

I measured the output and without the cap it has 1.1mv DC with it zero and to me there was no difference in the sound.

The 25k pot would probably be better. I adjusted the feedback loop resistors to 51k and 33k this gives more than enough gain.

I am actually designing a discreet circuit, those calculations for the resistors take some time to work out though. :D
 
Lose the 15k input shunt resistor or replace it with a 100k if you prefer and see if that fixes the level control quirk. You already know I think the output blocking cap redundant, the best sounding cap is no cap. The next upgrade will be a higher grade level control.
 
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Russellc said:
http://www.diyaudio.com/forums/pass-labs/103050-jfet-boz-2.html#post1226773

http://www.diyaudio.com/forums/pass-labs/103050-jfet-boz.html#post1226446

J-fet BOZ in a similar "case"............



Russellc
Click to expand...

Russellc,
Those DIY Audio threads are awesome, but not user friendly. Please help me cut to a base line. Is the J-fet BOZ a preamp with gain, or a gain module for use with a buffer like the B1, or both, or what? I spent an hour or two looking at it yesterday and I still don't know.
 
skywave-rider said:
Russellc,
Those DIY Audio threads are awesome, but not user friendly. Please help me cut to a base line. Is the J-fet BOZ a preamp with gain, or a gain module for use with a buffer like the B1, or both, or what? I spent an hour or two looking at it yesterday and I still don't know.
Click to expand...

As I understand it, it has plenty of gain. look at the original Bride of Zen
material...this is just a variation using j-fets instead of mosfets...sort of like the Aleph compared to the Aleph J.....They just replaced the 610 IRF devices with J-fets. These J-fets are way sonically superior to the op-amps, and by no small margin, just trickier to implement...but when you have Mr Pass's ideas you can rest assured they are good, well thought out ones!

Russellc
 
These things just need ca 10-15dB gain tops unless the power amp in use is really sub-standard insensitive.
 
Just a IMHO-

I have used both the AD823 and the OPA2134 in both phono pre-amps and pre-amps. My personal preference is the 2134. It is smoother in the mids (especially vocals) and has a tad deeper bass. BUT- it is a flavor choice. As they are inexpensive, try both!
 
Pio1980 said:
These things just need ca 10-15dB gain tops unless the power amp in use is really sub-standard insensitive.
Click to expand...

Remember, the gain*bandwidth product is a constant. Therefore as the gain lowers the bandwidth expands by the same factor. Just saying that extra bandwidth is not a bad thing.

A gain of 6dB is not bad. Most sources need attenuation not gain.
 
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What about the Pass B1 Buffer Preamp?

Fairly low to no gain?

Looks really interesting and 2SK170's are available from Mouser.

Sounds like a good idea as in both my systems I only need to turn up the volume to around 9.00 o'clock. A volume control without the drawbacks of a passive pre.
 
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There's a very nice implementation of the B1 for sale on Barter Town. I happen to be auditioning it right now. You're right, lower gain. But a very nice sound.
 
HypnoToad said:
What about the Pass B1 Buffer Preamp?

Fairly low to no gain?

Looks really interesting and 2SK170's are available from Mouser.

Sounds like a good idea as in both my systems I only need to turn up the volume to around 9.00 o'clock. A volume control without the drawbacks of a passive pre.
Click to expand...

Sorry for jumping on your thread like this.
I am building one currently, almost all the parts are here.
RussellC built like 15 of them.
If you have enough level running your sources straight to power amp, the B1 would do it for you, as it would only provide attenuation. I have no idea how it sounds, but I like the philosophy behind it.
 
skywave-rider said:
Sorry for jumping on your thread like this.
I am building one currently, almost all the parts are here.
RussellC built like 15 of them.
If you have enough level running your sources straight to power amp, the B1 would do it for you, as it would only provide attenuation. I have no idea how it sounds, but I like the philosophy behind it.
Click to expand...

No problem this is what's it's all about.

More than enough gain, if I connected my phono pre to my power amp I think the roof would lift off.

I have the order for Mouser done already, just need to hit okay.:D

Hopefully RussellC will chime in with his thoughts on the B1.

I am seeing it this way, my phono stage is fine, my power amp is fine and all my preamp is doing is getting in the way.

I have one question though, am I better getting two single ganged pots or going for a stepped dual gang, I know which way is cheaper.:D
 
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The DIY Audio thread for the "hot rodded" version of the B1, which I am putting together, is here:
http://www.diyaudio.com/forums/group-buys/147075-gb-dc-coupled-b1-buffer-shunt-psus.html
The group-buy thread where I got my PCB and parts bundle (from member Teabag) is here:
http://www.diyaudio.com/forums/group-buys/147075-gb-dc-coupled-b1-buffer-shunt-psus.html

EDIT:
Here are some good build threads with pics showing everything:
http://www.diyaudio.com/forums/blogs/tea-bag/296-salas-dcb1-blue-edition.html
the following is step by step:
http://www.audiocircle.com/index.php?topic=85001.0

DOUBLE EDIT:
There are guys still building the "standard B1." That can run off of a wall-wart and does not need heat sinks IIRC. Those boards are available at the PASS DIY store. Why Hot Rod Hypnotize? Since I have not heard either, I can't tell you. I am doing this in a leap of faith. Should be fun if I don't smoke the components.
 
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The pot is why I am almost done collecting parts.
I have tried the cheap stepped attenuator I referred to earlier. And I just put in an Alps (in my current old preamp, which needed a new one anyway.) So I am using that for demo-ing pots. But I won't go exotic, that's not in the budget.

I am going to ask Westend to put together a case for me when I figure the layout.
 
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