Stand alone gain stage pre-amp

[mantha3]

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!

 

[rongon]

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.

 

[Frank Berry]

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.

 

[mantha3]

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?

 

[mantha3]

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.

 

[mantha3]

How about a full on PreAmp board in place of the basic OpAmp. This is just 9 bucks

The NTE721 preamp circut.

NTE721

 

[kevinkr]

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..

 

[kevinkr]

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.)

 

[dgta]

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.

 

[mantha3]

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...

 

[mantha3]

So should I buy two OPA627 OPAMPS and run 1 per channel?

 

[Joshua_G]

IMHO, opamp based gain stage is way easier to build and way cheaper. However, when it comes to sound quality, tubes based gain stages win hands down.

Salas 6V6 pre-amp should be a great choice sound-quality-wise and as easy and economical to build as it can be for a tubes based pre-amp.

 

[mantha3]

I am still paying for my monoblock tube amps... Need to go cheap and run the OPAmp

I did some research and found the "Rod Elliott High Quality Audio Preamp (MkII)"

Rods HQ Pre Amp

This is 2 stages... I am newer to this so I drafted up a dummy version of the design Rod has... Just the 2 stages with the Volume Pot between the stages as he designed.

Do I have the Volume Pot. right? The 3 solder tabs?

 

[Joshua_G]

1 OpAmp based gain stage will be more than enough for you, if you don't need the balance control.

 

[Frank Berry]

The output stage alone should be enough. I've added a bit of gain by changing R7L from 10k to 2.7k and removed R6L.
R5L can also be removed and replaced with a 1uf capacitor.

 

[mantha3]

Sorry,

Better photo.. So, Since I'm running a passive preamp... Thinking of just doing away with the Pot on this... Have this just be a gain stage and then use the 100k Attenuator on the passive. Think that works?

Do I need the Capacitor on the signal input?

 

[Joshua_G]

Better photo..

It should work well.

So, Since I'm running a passive preamp... Thinking of just doing away with the Pot on this... Have this just be a gain stage and then use the 100k Attenuator on the passive.

Once you have an active pre-amp, it's much better to skip the passive pre-amp and have only 1 unit, which will include the source selector, volume control and gain stage.

 

[Frank Berry]

Your schematic is wrong. You have grounded the + input of the opamp.

 

[mantha3]

Frank Berry,

Check out Rods HQ Pre-amp

I think my schematic is going off of what he has in Figure 4

I appreciate you taking the time to look at this with me by the way. I owe you all a beer.

 

[Joshua_G]

Your schematic is wrong. You have grounded the + input of the opamp.

You are correct.
Corrected schematic is attached.