• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

DIY Simple Active Preamp

Really nice looking amp, as far as the pop, it's perfectly normal, to avoid this you would have to install a relay circuit.

To keep everything simple I didn't bother with this, but do what I do and turn on the preamp first and the power amp last. When powering down turn off the power amp first and preamp last and there will be no problems.

If you want to measure DC offset, set your DMM to at least 200mv's. Put the positive lead on the right or left positive output of the preamp and the negative lead to the negative output. It should read zero if you have the output caps installed.

Before connecting any of my projects to other gear I always check for DC offset at the outputs and inputs just to make sure I don't have an "oops" and destroy something.
 
Last edited:
sw-r

Beautiful casework, and assembly. But no pictures of the goodies! Please post the inside.

I would guess, just a guess, that your case cost more than all the insides. But I would also say it was money well spent.

The turn on DC leak through, the thump, can be avoided by not turning on your power amp for 10s-30s after powering up the preamp. Or put in a relay and a delay circuit. One is cheap, the other foolproof.
 
Last edited:
Grainger49 said:
sw-r

Beautiful casework, and assembly. But no pictures of the goodies! Please post the inside.

I would guess, just a guess, that your case cost more than all the insides. But I would also say it was money well spent.
Click to expand...

I was drooling over the case as well, imagining what I could fit inside.:yes:
 
Yes sw-r, very nice case. Kudos for being able to get it all assembled w/o getting a scratch or a mark on it. Commercially available or DIY? Tell us more.

I'd like to get your impression, and more info about your step attenuator. Do you still have the item# from fleabay?

Looks like you have plenty of room so I would go with a relay circuit to resolve the thump. You won't have to remember the turn on/off sequence.

Enjoy - - -
.
 
ghazzer said:
...I'd like to get your impression, and more info about your step attenuator. Do you still have the item# from fleabay?...
.
Click to expand...

+1. I too am interested. Is it a smooth operation when increasing/decreasing volume or are there any pops between settings?

Please do show nudies of your project for others to compare. Looks great!
 
Thank you guys very much.

I am at work, but as soon as I can when home I will post the info for the attenuator. (Actually, I remember I posted it back in this thread when HypnoToad was talking about pots.) There are no pops. I tried one on my old PS buffer type pre, and the step to step accuracy really makes a tight center. But in the end I went with the Alps Velvet in the PS preamp because I thought the sound was a bit rounder.

For this, I will leave the stepped attenuator in.

I will post pics of the interior and listening impressions at some point, but I have been too busy to listen much.

This pre is built into a Par Metal 1 rackspace chassis because it will eventually go into a rack housing a DEQ 2496/DCX 24/96 active setup for my Altec 9844-8Bs. Those are in my office at work mostly, but are rolled out for hi fi playback of student sound projects and small screenings. That's why I spent a few extra bucks on the rack chassis.

I had an intermittent short in the power switch, maybe I applied too much force when tightening in down, or too much heat when soldering. It could also be a cheap crap part, but luckily I had another waiting for my B1 Hypnotize pre, so I swapped that in.

One thing that bothered me from the beginning of this build is that I did not know what to do with chassis ground. In the past, ancient times, I think equipment would have separate signal and chassis grounds. But I did not know what to do here. So this is what I found so far:

I get a buzz when connected to my system on any amp. I don't on my PS pre. I suspected my homebrew +/- supply, so I rigged 2 9V batts, and the buzz is identical. My power supply is not grounded. So I ran a wire from the SSP chassis to one of the RCA colds on the PS pre and the buzz went away. Then I ran it to an AC ground and same result. So....I will rewire a 3 prong AC cord to my PS. I checked for continuity to ground along all points on the PC/connectors/pot and chassis, through to the power supply, and I have ground. So I may have a question for the experts when I open up my supply on how to wire the cord correctly. (I built it 10 years ago for IC projects.)

The SSP has TONS of gain. I am used to a buffer type pre at home, so....it's a lot of oomph.

Additionally, I measured DC offset, and after double checking, I find NONE. I went through all the ranges on my meter. Hard to believe so I will triple check when I have time.

Thanks to all for the project and help. More later.
 
Last edited:
skywave-rider said:
The SSP has TONS of gain. I am used to a buffer type pre at home, so....it's a lot of oomph.

Additionally, I measured DC offset, and after double checking, I find NONE. I went through all the ranges on my meter. Hard to believe so I will triple check when I have time.
Click to expand...

It doesn't have a lot of gain but it's all relative as a passive pre has negative gain and the Pass Buffer has zero. I will check mine on the scope to see exactly how much but the calculations are 1.7db.

If you constructed it right and with the output caps in place you should have zero DC offset. If you change the 8.2k resistors to 10k it would have zero gain or what they call unity gain.
 
Yea, since I normally use a buffer type pre at home, it seems like tons! :D It would be interesting to see what the actual performance is.

Thanks for explaining why I have zero DC offset at the output connectors. I will remeasure upstream of that if I can easily get there with a probe.

I think unity would be good, it's good to know the resistor option to change the output level. But I will probably leave it. Buzz is the priority.
Then get it in the rack for Phil Spector day.

Thanks.
 
skywave-rider said:
I am at work, but as soon as I can when home I will post the info for the attenuator. (Actually, I remember I posted it back in this thread when HypnoToad was talking about pots.) There are no pops. I tried one on my old PS buffer type pre, and the step to step accuracy really makes a tight center. But in the end I went with the Alps Velvet in the PS preamp because I thought the sound was a bit rounder.
Click to expand...

Is this the unit you went with (no affiliation):

http://www.ebay.com/itm/190550915869?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1438.l2649
 
Light speed Attenuator

Hi,
If anyone is looking Lightspeed Attenuator for volume control,just let me know.I will have few boards with matched LDRs.It is suppose to be better than Alps,or any other volume control.

Regards,
Sachin
 
HypnoToad said:
It doesn't have a lot of gain but it's all relative as a passive pre has negative gain and the Pass Buffer has zero. I will check mine on the scope to see exactly how much but the calculations are 1.7db.

If you constructed it right and with the output caps in place you should have zero DC offset. If you change the 8.2k resistors to 10k it would have zero gain or what they call unity gain.
Click to expand...

I think with 8.2K and 10K feedback, Vout = 2.23 X Vin
With 10K and 10K as feedback resistors, Vout = 2 X Vin. I am using later one.
 
I put the SSP into the rack at work and it's running w/o hum. Currently on batts. Will use it in a classroom for some playbacks on Tues..

Not much listening yet, but I am probably going to swap-out the stepped attenuator with either an Alps (can't find 25kOhm value,) or a lightspeed attenuator.

I feel there is some of the character I did not like from the EBay stepped attenuator as when I had one in my old preamp. We'll see when I put the setup into a big room on Tues.

I can buy a 50kOhm Alps. Is it a good idea to put resistances in parallel to get 25kOhm?
 
Last edited:
All's good. Here's the pre-amp in action in the rack:
attachment.php


And here's some students making a tape loop (with 3 machines) with one of the Altecs in back.
attachment.php


I'm glad I made the SSP. It's doing its job well. Thanks to those involved with the project. :thmbsp:
 

Attachments

  • IMG_9065.jpg
    IMG_9065.jpg
    68.5 KB · Views: 485
  • IMG_9058.jpg
    IMG_9058.jpg
    74.9 KB · Views: 588
You must log in or register to reply here.
Back
Top Bottom