Accademic question about standalone preamps and SAG sensing

[kagliostro]

In an italian forum there is a guy asking about to build a separate preamp and final amp

he is asking about differences in behavior of the same circuit in a whole amp and in splitted configuration

So a question come to my mind

Having separated PS there is a way to have a preamp circuit that is sensitive to the SAG present on the Power Amp ?

there is a way to have a connection as to give a feedback sense to the preamp toward the Power Amp ?

I know this is a Crazy Question, but curiosity is much

Thanks

Kagliostro

[AZJimC]

If you mean power sag, then there is a way. A few older amps were built in separate chassis, but they used an 8 pin socket and cable, or other connector, to transfer high voltage from the power amp to the preamp. this way the preamp needed no transformer, and it was as if the unit was all together in a single cabinet. AFAIK these were used primarily in old organs etc. I haven't seen a set up with actual split units, other than in a few audiofile stereo amps using tubes.

[HotBluePlates]

I've never probed preamp supply voltages while playing an amp to see if the voltage dropped due to power amp current. It would be interesting to find out if any noticeable drop occurs, given how many smoothing stages an amp typically has, and that the preamp pulls very little current from the supply when passing signal. I would guess that there's little sag to the preamp, regardless of what's happening in the power amp.

Anyway, I'm building an amp now that is like what you and Jim are describing; that is, a separate preamp and power amp chassis. The power amp's portion of the power supply is in the power amp chassis, with 2 cables connecting to the preamp chassis: one for power supply voltages, and a second for signal.

I'm using a 6-conductor shielded cable for power, and a 2-conductor shielded cable for signal. The 6-conductor transfers B+, ground, a twisted pair of filament voltage and a twisted pair for the power switch. The 2-conductor transfers the signal from the phase inverter outputs, with the coupling caps  in the power amp chassis.

I'm using XLR-type connectors. I'm connecting the shield of one cable to the shell of the connectors at both ends, with the other cable only connecting at one end, as typical of shielded cable. That's because I have the amp's ground isolated from the chassis to allow use of a true ground-lift switch, while keeping the wall cord ground wire attached to the chassis for safety. So the one cable that connects the shield to the connector shell (and therefore the chassis) at both ends is bonding one chassis to the other. The 6-conductor's ground wire simply connects to the isolated ground in the power amp chassis to the isolated ground in the preamp chassis.

I'm keeping the preamp's power supply filter caps and dropping resistors in the preamp chassis, rather than putting all the power supply in one chassis.

[kagliostro]

Many thanks for infos AZJimC & HotBluePlates

I'm using XLR-type connectors. I'm connecting the shield of one cable to the shell of the connectors at both ends, with the other cable only connecting at one end, as typical of shielded cable. That's because I have the amp's ground isolated from the chassis to allow use of a true ground-lift switch, while keeping the wall cord ground wire attached to the chassis for safety. So the one cable that connects the shield to the connector shell (and therefore the chassis) at both ends is bonding one chassis to the other. The 6-conductor's ground wire simply connects to the isolated ground in the power amp chassis to the isolated ground in the preamp chassis.

I'm keeping the preamp's power supply filter caps and dropping resistors in the preamp chassis, rather than putting all the power supply in one chassis.

those are very interesting things, I can well understand the reason for the filter caps and resistor (those of the preamp circuit)  to be put in the preamp chassis instead near the PS on the PA chassis

Sluckey in one of his amp (a Vox remake) has a separated preamp and PA circuit with an octal socket to have the signal and B+ connected from one chassis to the other (on the classic way), but I don't know how he do it exactly

do you think XLR are good for B+ transfer ?

XLR will be a good choice because they are easily available

-------

For ground lifting what do you think about the use of this circuit ?



from this Merlin's .pdf file

http://valvewizard2.webs.com/Grounding.pdf

Kagliostro

[jjasilli]

I'm thinking that if the power tubes cause a voltage drop (sag), this will also drop the B+ supply downstream to the preamp.  However, a 30 - 60V B+ drop to the preamp will probably be insignificant, unless the preamp voltages are low to begin with - so that preamp plate voltage drops down to 100VDC or less. 

[sluckey]

Having separated PS there is a way to have a preamp circuit that is sensitive to the SAG present on the Power Amp ?

there is a way to have a connection as to give a feedback sense to the preamp toward the Power Amp ?

Let's see if I understand the question correctly...

You have a separate preamp and power amp. Each unit has a totally separate power supply. You want the preamp power supply to sag just like the power amp power supply.

Yes, this can be done (See the attached pic). You do need a sample of the power amp B+ to act as a sense input to a variable voltage circuit. You can use a standard VVR circuit with a slight modification. Connect the VVR to the preamp B+ line, but connect the voltage level potentiometer to the B+ sample from the power amp. That's all you need to do. Now when the power amp B+ sags, that sag will be felt at the gate of the FET and will cause the drain/source resistance to increase, dropping (sag) the preamp B+ voltage as it passes through the FET to the output. IOW, the VVR output will have the same sag characteristics as the power amp B+.

Now I gotta ask,,, WHY? You must connect the power amp B+ to the preamp chassis to sense the sag. Why not just apply the power amp B+ to the preamp filter caps/dropping resistors and be done with it. You get the same results and have enough money left over to buy a couple really nice steaks.

Here's the circuit. I named it rube.gif. I think the name is appropriate.    

[jjasilli]

Now I gotta ask,,, WHY?  Yes, that's the question!  Perhaps implied in my prior post.  BTW:  I go the opposite route & put a diode after the screen supply to "rectify" B+ to the preamp -- to prevent power amp sag from reaching the preamp, especially for transients.  The purpose is to preserve transient spikes of signal to compensate for power amp sag and the "sponginess" of chokes (which I adore).  I.e., that diode is the poor man's alternative to a separately powered preamp.

[kagliostro]

Thanks Sluckey

your circuit is what I was asking for !

As I told this is a crazy question and come to my mind after I've read a post in an italian forum, a guy was asking about the difference that there is having a whole (preamp + PA) amplifier and a standalone preamp + separated PA

As he want to have a complete stand alone preamp (with a proper PS) to be used also without the PA and he asked about differences with the splitted circuit, to me seemed clear that different amps with different PS will act in a different manner, in those with a very good PS the B+ voltage will not Sag at the preamp, but in those amplifier that has a poor PS I think there will be Sag also to the preamp B+ and this will contribute to the particular tone of the amp

so my crazy question and your sly solution

I must also say that what HBP and Jjasilli say about the fact that sag is poor or have no much effect on the preamp, has sense

Thanks

Kagliostro

[kagliostro]

I go the opposite route & put a diode after the screen supply to "rectify" B+ to the preamp -- to prevent power amp sag from reaching the preamp, especially for transients.

I know the use of that diode, it is often used in HiFi plans and has its reason to be a good thing

the question , however, was implying  the fact to split in two a "classic" amp and was a research to preserve original characters

Kagliostro

p.s.: I was writing the same time with Jjasilli

[HotBluePlates]

do you think XLR are good for B+ transfer ?

XLR will be a good choice because they are easily available

-------

For ground lifting what do you think about the use of this circuit ?

Well, there's a catch to that. XLR connectors seem to be typically rated for 50v. However, if you look at the full technical specs, the inserts are rated for a dielectric strength of 1.5kV. You wouldn't want to put any voltage that high through them, because of the relatively close spacing of the pins, especially in the 6-pin format.

However, ~300v is a different thing than 1.5kV. I have a pair of tube microphone preamps that a very well-respected audio builder made for me. They use a separate power supply chassis, which feeds the 250v B+ and 6.3vac to the preamp chassis via a 4-pin XLR. So I know they work in practice. As a result, I'm not afraid to use them in this situation.

And yes, I'm using the circuit you drew for the "ground lift" setting. It maintains a ground connection for RF, while lifting the circuit ground from earth ground. The 3-wire power cord always keeps its earth ground solidly connected to the chassis; signal ground simply gets connected directly to earth ground, or through the ground-lift network.

[Willabe]

And yes, I'm using the circuit you drew for the "ground lift" setting.

With or without the antiparallel diodes?


                    Brad     

[kagliostro]

Thanks for explanation HotBluePlates

@ Willabe

I think with, to me seems that those diodes are indispensable for that circuit

HotBluePlates will confirm or not my supposition

Kagliostro

[HotBluePlates]

Without the diodes.

Different folks draw the circuit differently. Jensen (transformers, not the speakers) shows a parallel resistor and cap for a ground-lift circuit. KOC shows a series 51 ohm resistor and cap for his ground lift circuit. Either way, neither uses the anti-parallel diodes.

I don't want to cause a potential difference between chassis ground and circuit ground in the ground-lift mode (which I see the diodes doing); I want to isolate the circuit ground from chassis ground, while providing a low-impedance path for RF-induced buzz. I'm thinking KOC's method may suit my goals best (though, as always, I defer to those who've tried these before, which I have not).

[kagliostro]

Oh, OK HBP

the more circuit of this kind you've seen are without diodes

those I've seen are with diodes, so I was convinced that they were indispensable  



Quote from Pete Millet

I isolated the AC safety ground, which is connected to the AC plug ground and the chassis, from signal ground using a "ground breaker" circuit.  This is an arrangement of series / anti-parallel diodes that presents a high impedance unless there is a voltage of more than two diode drops (about 1.4V) across them.  These diodes are large - they must be sized such that enough current can flow through them to cause the fuses to blow if there were to be a short between signal ground and the AC line.

Pete Millet amp http://www.pmillett.com/DCPP.htm

one other version with diodes (a bridge)



from here http://sinhalacurcuit.blogspot.com/2010_01_01_archive.html

there are really many way to do things

(though, as always, I defer to those who've tried these before, which I have not)

as you say the better is to test it, I'm in the same condition in which you are

----

your intention is to put a switch in parallel with this "ground breaker" to insert or exclude it ?

Kagliostro