Another quickie... theory

Hi all...

I've been looking at some solutions on designs in tube mic preamps and I must clarify a few things for myself and I would like to get the answers from you guys vast experiences and knowledge.

Anyhow... looking at a mic input transformer and desired input impedance I know that the input impedance is a reflection from the load at the secondary.

I also know how to calculate parallel resistor values but my question is how the in this case third resistor (R3) is affecting the other two (R1 and R2)?

Since it is AC and the capacitor allows AC to pass I guess the third resistor is in the calculation as well? Why using the cap and the third resistor? Well, that's one of the thing to understand as well.

Forgive me if this is too simple

Best regards

// John

For calculation, you assume the capacitor is a short circuit. The load will vary with pot wiper position.

JR

Thank you Sir

In fact the behaviour of the wiper was another concern.

Can you tell me if I can't find a suitable pot value and would like to alter the total resistance value of the pot if there is a way to keep the (in this case audio taper) and still alter the total resistance?

Best regards

// John

Would this keep the taper of the pot? Or is there something else happening under this connection?

best regards

// John

Sorry I do not understand what you are asking..

JR

Oh sorry my bad...

I'll try again.

I've read a lot on the internet about pot behavior from loading and bridging with resistors.

The input transformer drawn is for real with a 1:10 ratio and I would like to have a 150k pot across the secondaries... finding that pot with solder lugs seems tricky as far as I know so I began thinking about bridging a 250k pot to achieve the desired 150k... this I can do even with a 250k pot with 20% tolerance.

Then I realized that it might mock up the behavior of the pot and thereby the vague question... what will happen with the pot behavior and perhaps how to avoid unwanted effects.

Best regards

// John

johnheath said:

Oh sorry my bad...

I'll try again.

I've read a lot on the internet about pot behavior from loading and bridging with resistors.

The input transformer drawn is for real with a 1:10 ratio and I would like to have a 150k pot across the secondaries... finding that pot with solder lugs seems tricky as far as I know so I began thinking about bridging a 250k pot to achieve the desired 150k... this I can do even with a 250k pot with 20% tolerance.

Click to expand...

Yes....

Then I realized that it might mock up the behavior of the pot and thereby the vague question... what will happen with the pot behavior and perhaps how to avoid unwanted effects.

Click to expand...

but any resistance to ground from the pot wiper will also affect the total load.  This can be calculated by considering the pot as two resistances (from wiper to either end) with one in parallel with the wiper load.

You can probably dial this in for nominal 50% rotation of an audio taper pot (nominally 10% resistance). Load deviation from this nominal will result in some modest, perhaps inconsequential load impedance change.

JR

Best regards

// John

Click to expand...

Thank you sir

Yes I get that it will alter the overall resistance.

If you look at the second picture I posted... what could be said about the subject in that case?

Best regards

// John

It would be better for us to understand your goal.  Why 150K?  Usually not critical.  I also question the use of a 1:10 immediately followed by a voltage divider.  You normally use a 1:10 because you need a lot of ‘starter’ gain before even more gain. 

You want to aim, for example, for the total load to be as close to 150K at all times. The way to do this is to make anything hanging on the pot wiper much bigger than 150K.

So suppose you use a 250K pot and add a parallel resistor so the total becomes 150K i.e you need a 390K . You probably want to add a resistor from the pot wiper to 0V so that the grid of the input tube always has a DC connection to 0V even if the wiper becomes disconnected. In fact it is a good idea NOT to include the pot in the DC  because it can induce scratchiness so you will probably include a capacitor in series with the wiper. Now you have to add a resistor from the other side of the cap to 0V to endure the tube grid has a DC path to 0V. You might typically use a 1Meg resistor for this. With the wiper at the top, the worst case, the 150K is reduced to  130K which is a 20% variation and in osxt cases will be perfectly acceptable. You could bootstrap the imput to the first stage which could raise its effective input impedance by 10 times to 10Meg or more in which case the variation in the 150K would drop to a little over 1%.

I should point at that as a general rule, placing a pot right after the input transformer is not a good design choice for a tube mic pre. The reason is that at any position of the wiper other than at the top, the noise performance is seriously degraded by the added series resistance. A better approach would be to place the pot after the first stage where noise should no longer be the major issue, or vary the gain of the fist stage rather than attenuate the signal.

Cheers

ian

Thanks a lot both of you guys

Yes I know that placing the pot right after the input transformer is probably not the the best solution... Normally I place it between the gain stages. Thing is that I am experimenting with a topology that need a quite high grid resistor before the second gain stage and I don't want to use a pot with that high resistance (500k-1MEG) in that position. The solution could be using the pot before the first gain stage... but, as you already have figured out I have some concerns about it.

Unfortunately I do not have any possibilities to experiment with noise figures where I am right now so this is in the thinking stage of producing.

So my original intention with my (vague) questions is to properly understand the behavior of the pot and the interaction with the loading resistors.

I could of course experiment trying to use as low value as possible of the pot when placing it between the two gain stages?


Best regards

// John

Pot after input transformer has been done.  Gates did it.  Universal Audio did it.  Probably more that don't come to mind as quickly.

mjrippe said:

Pot after input transformer has been done.  Gates did it.  Universal Audio did it.  Probably more that don't come to mind as quickly.

Click to expand...

gates doesn't count.
what UA product did it ?

gridcurrent said:

gates doesn't count.
what UA product did it ?

Click to expand...

Not a tube unit, but didn't the 1108 essentially do this with its "inputrim" control? It was a FET input, though, so high impedance.

rackmonkey said:

Not a tube unit, but didn't the 1108 essentially do this with its "inputrim" control? It was a FET input, though, so high impedance.

Click to expand...

Exactly.  And who says Gates doesn't count?  They made a lot of fine gear.

Another one:
http://www.pmillett.com/micpre.htm
https://groupdiy.com/index.php?topic=72268.msg918190#msg918190

rackmonkey said:

Not a tube unit, but didn't the 1108 essentially do this with its "inputrim" control? It was a FET input, though, so high impedance.

Click to expand...

similar to "vaporware".
as popular as the Ampex 1" 3 track  MR-70.
seen more 1108's than I care to remember, zero with the LDR.

Another example, RS124? Why doesn't STA Level count?

Squeaky said:

Another example, RS124? Why doesn't STA Level count?

Click to expand...

Because we are talking specifically about preamps.  Tons of tube limiters used a pot after the transformer.  However gridcurrent has not addressed Gates.  Also the UA 1008 tube preamp had the LDR option, but as gc says, it was rarely used.  Just curious why the idea is shunned.  Having a varying impedance can have pleasant effects.

mjrippe said:

Because we are talking specifically about preamps.  Tons of tube limiters used a pot after the transformer.  However gridcurrent has not addressed Gates.  Also the UA 1008 tube preamp had the LDR option, but as gc says, it was rarely used.  Just curious why the idea is shunned.  Having a varying impedance can have pleasant effects.

Click to expand...

My apologies, I should have paid more attention.