DIY Simple Active Preamp

nj pheonix said:
Ok tamped down the input . 10k in series 15k to ground (I would've used 12ks as suggested but I had none) . Not quite enough gain. I had rg at 33k. Now that I'm in experiment mode, I left leads long and paralleled 22k across for 13.2k @rg. I'd guess this is pretty close. Very good control of gain through the attenuator. I have listen a bit and see what I have now. Thanks all for getting me this far!
PS all the add ons make this pretty ugly as it sits. Once its dialed in it'll have to be cleaned up a bit!
Click to expand...

Without trawling through 400+ posts I'm not sure exactly what topology you're using. Do you have the volume control before the op-amp or after it?
If before the op-amp, why use two resistors to pad down the input? By doing so, you have reduced the input impedance considerably. Is that what you wanted to do? The method I suggested is neater and keeps the input impedance higher.
If the volume control is after the op-amp, is there any need for an active pre-amp at all? A 50k volume control after the op-amp is really just a passive pre-amp anyway (but with a higher input impedance).

I think I'll stop giving advice to people who ignore it.
 
The pot usually goes at the input. In the case of too much gain with an audio taper pot, the solution is to reduce the gain via adjusting loop feedback network.

Sent from my SM-N900V using Tapatalk
 
Pio1980 said:
The pot usually goes at the input. In the case of too much gain with an audio taper pot, the solution is to reduce the gain via adjusting loop feedback network.

Sent from my SM-N900V using Tapatalk
Click to expand...

Yes, it usually does. However, I was inquiring about this particular pre-amp.
 
nj pheonix said:
If you look at attachments in # 458
You can see pot in circuit
Click to expand...

Ok, I get it now. I had looked at the circuit diagram but there is no pot shown. I can see it on the PCB track layout.
I would recommend one resistor in series with the pot to maintain a higher input impedance and avoid components duplicating each other. However, don't do that before dropping the gain to unity.
 
A local circuit genius, I'm not being snide, he is, loves the Multicap RTX polystyrene caps for passive EQ and for coupling caps.

We are buying them from Sonicraft and paying an additional $5 per pair for the exact value specified and matched. That makes quite a difference for RIAA EQ.
 
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Judas Priest said:
I am thinking of building another one of these in SMD. Would these capacitors be OK for the output?

http://www.reichelt.de/Folienkondensatoren/2/index.html?ACTION=3;ARTICLE=31430;SEARCH=SMD-2824N 2,2µ
Click to expand...

Hand assembling SMD components? That isn't a job for the feint-hearted! The advantage of SMD is the speed and efficiency of mass production by automated machinery. I don't think there are any benefits for audio quality other than consistent and compact pcb layout.

I agree that polypropylene and polystyrene caps are the best, though I wouldn't say which one of the two is better (maybe polystyrene).
 
Thanks for the thoughts everyone!

I am starting to shy away from SMD; it is just so tedious and time consuming to soldier the components. SMD boards are cheaper to have etched because they

are smaller and there are few holes to drill.


I think I´ll just stick with a conventional layout, even if it is a little larger and more expensive.



It is also easier to measure something on the board without shorting the pins of an IC :)
 
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My preamp has a slight 50Hz hum. I can only hear it when I have the gain on my power amp turned up alt least half way, with no signal. It is really not that audible from a normal distance, but it kind of bugs me.

Strangely enough it increases slightly as I turn up the volume on the preamp and completely disappears starting at about 80% volume. WTF??

Can anyone make sense of this?
 
Hum is such a fun problem to troubleshoot. The first question should be:

Can you hear hum from your listening chair during soft passages? If not try to ignore it. I can hear hum when I put my hear to the speaker. That isn't my listening position so it is a moot point.

But if the answer to the above was yes then it could be a ground loop or induced hum.

Induced hum is caused by the signal wires having AC induced into them. Any transformer near the cartridge or arm can cause this. Turntables should be 2 feet away from any power transformer.

Sometimes the phono stage gets hum induced into it by a power cord or transformer. Again distance is your friend here to keep hum put of the phono pre. It should be shielded too.

Your odd volume related symptom has me stumped.

If the problem is a ground loop, that is another post of general practices.

Good luck, post back.
 
Judas Priest said:
My preamp has a slight 50Hz hum. I can only hear it when I have the gain on my power amp turned up alt least half way, with no signal. It is really not that audible from a normal distance, but it kind of bugs me.

Strangely enough it increases slightly as I turn up the volume on the preamp and completely disappears starting at about 80% volume. WTF??

Can anyone make sense of this?
Click to expand...

The pot has an output impedance which is dependent on its setting. The highest output impedance is with the wiper at the electrical half-way point. That point is at about 8/10 for a log pot.
Is the pot mounted on the PCB or is it on flying leads? If on flying leads the wires should be sheilded.
 
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I´ll have to check tomorrow if the hum is audible from my listening position; I think it might be (slightly). I could just ignore it, just as I ignore my loud tinnitus, but it bugs me from a technical standpoint.

I only use an (a?) USB DAC as a source, no turntable. I tried hooking up a CD player directly to the amp (bypassing the pre) and there is no hum.

I´m using a 10K Alps log. pot, with flying leads. I am probably going to build a new preamp/Linkwitz-Riley crossover and put it all in one case. I am trying to avoid making the same mistakes twice :) Thanks for the tips!
 
There should be a Tinnitus Club thread here. I feel your pain.

If the Simple Active is not in a metal case this could be the problem. But the flying leads by themselves shouldn't make that much difference.

I still have an unused Black Beauty Alps pot. It should be in my system but I am reluctant to mess with something that sounds so good.
 
My Pre is in a metal case. I have the case grounded (mains ground/earth); I removed the ground connection to see if it made a difference; it didn´t.

Since the hum isn´t that audible, if at all, from my listening position, I´ll just leave things as they are. Maybe when I build the next one there will be no hum :)
 
This is a DIP adapter and you have to use it for any other opamp, even for SOIC type

Regards
Sachin
 
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