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Old 6th October 2011, 06:30 PM   #1
mantha3 is offline mantha3  United States
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Default Stand alone gain stage pre-amp

I run a Tube Amp based system. I run with a passive preamp. The system works well for me for the most. I have a good CD Player with a good signal to drive a passive pre based system. I also have the same for digial DAC stuff.

I have a Motorola Digital Converter box for my Cable TV service. Kinda junk. If I have my preamp set at like 50% and I’m listening to the CD player the music is loud and dynamic .. I switch over to the converter box I need to adjust the vol to about 70% to get the same volume and dynamics… It just seems like I could use a gain stage between this converter box and my passive.

So, I was thinking of building some form of simple gain stage with a Potentimeter. So the pot could be adjusted to an ouput of 2volt up to 5 volt output. Some sort of Simple Class A gain stage… 12AU7? Pot to adjust the overall gain. This is sort of like the stage built into a nicer CD player output I think.

So that is my thought… Wondering if anyone has knowledge of a design or a DIY product / kit that sounds like a good fit for this. I’d like to keep it cheap but would entertain any and all ideas. I’ve built Amps, passives, speakers… I’m not educated enough with the hobby to design.

Thanks DIY-ers!
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Old 6th October 2011, 07:02 PM   #2
rongon is offline rongon  United States
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Quote:
So the pot could be adjusted to an ouput of 2volt up to 5 volt output.
That's a gain of only 2.5X. The lowest gain that would be easily obtainable from a small triode would be about 12X. A lot of that gain would need to be knocked down by the pot. Maybe you'd want to use a pot to measure what voltage divider values (two resistors per channel) you'd need to adjust the output from the digital converter box to equal that from the CD output. In that case, most of the medium mu dual triodes would be useful, such as 6SN7, 6FQ7, 6N6P, 5687, and so on. 12AU7 would work too, but is thought to have worse distortion than the others. Put a voltage divider at the input to knock down some of the output from the cable converter box, so that the output from your tube gain stage roughly equals the output level from the CD player.

Since the audio quality from the converter is already somewhat compromised, I'll bet most would recommend a good quality dual opamp and set that for the required 2.5X gain. Much cheaper, low voltage, etc.

Anyway, that's my (humble, probably ill-informed) opinion. I hope it helps.
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Old 6th October 2011, 07:14 PM   #3
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I would use a dual opamp as well. The output of these cable boxes are, for whatever reason, far lower than audio industry standards. The Scientific Atlanta cable converter boxes also have a low audio output level.
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Old 6th October 2011, 08:18 PM   #4
mantha3 is offline mantha3  United States
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I don't know if the sound is compromised.. The signal is good.. It sounds great.. Just strange how much lower the gain is from other components I have connected to my passive preamp.

I've just been reading about Low Power Dual Operational Amplifiers. Interesting stuff. I like the theory on these. Cheap enough where I may just give this a run for the heck of it as a test. Spice 4 old RCA pairs of wires for an input / output in stereo...

So I'm thinking
I'd connect the postive of the input RCA L Channel to pin 3
I'd connect the GND of the input RCA L Channel to pin 2

I'd connect the postive of the input RCA R Channel to pin 5
I'd connect the GND of the input RCA R Channel to pin 6

I'd connect the postive of the output RCA L Channel to pin 1
I'd connect the postive of the output RCA R Channel to pin 7

I'd connect the GND of the output RCA L Channel to the GND of the Input RCA L Channel
I'd connect the GND of the output RCA R Channel to the GND of the Input RCA R Channel

Then I just control how strong in volts the output signal is by how much DC volt goes into Pin 8 (+) and Pin 4 (-). Perhaps just use 2 AAA battery which would be 3 volts.. Then 3 for 4.5 ect? Till the signal is strong enough?

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Old 6th October 2011, 08:24 PM   #5
mantha3 is offline mantha3  United States
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PS - I appreciate the lead on a cheap solution... If you have any specific dual Opamp that would be good to go with let me know. These all look the same when I go Google.
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Old 6th October 2011, 08:45 PM   #6
mantha3 is offline mantha3  United States
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How about a full on PreAmp board in place of the basic OpAmp. This is just 9 bucks

The NTE721 preamp circut.

NTE721
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Old 6th October 2011, 08:50 PM   #7
kevinkr is offline kevinkr  United States
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You need to read up on basic op-amp theory first. Read one of the applications notes or spec sheet for a typical dual op-amp..

This should help a lot: http://sound.westhost.com/dwopa.htm

Look at figure 3, this is what your basic circuit would consist of, one required per channel. Power could come from a pair of 9V batteries..
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Old 6th October 2011, 08:51 PM   #8
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by mantha3 View Post
How about a full on PreAmp board in place of the basic OpAmp. This is just 9 bucks

The NTE721 preamp circut.

NTE721
Wrong choice, this is intended for very high gain, low level applications, and in any event is not a complete solution - it is just a plastic dual in line package IC - AKA PDIP or DIP. (Used for phono amps, tape amps, and microphone amplifiers.)
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Old 6th October 2011, 09:36 PM   #9
dgta is offline dgta  United States
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Quote:
Originally Posted by mantha3 View Post
I need to adjust the vol to about 70% to get the same volume and dynamics…
OK, so what's wrong with adjusting the pot to 70%? Why do you need anything else?

In an ideal world, you would have a gain structure that has you dial the pot to 99% at the loudest point you'll ever use.
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Old 6th October 2011, 10:37 PM   #10
mantha3 is offline mantha3  United States
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Kevinkr,

I appreciate you responding and helping me out. I did look at the Figure 3.. I'm not getting where the 9V (+) and (GND) of the battery connect... I think the IN and OUT are the audio signal...
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