Separate passive pre and monoblocks?

[DrewP]

In my PP amp build I'm finding that I'm running out of space (moving bits around chassis, not constructing yet).

Amp has separate power trannies for each B+, plus a bias supply tranny, output tube filament trannies for each channel and a shared filament tranny for the input tubes. (overkill, but they are the trannies I had access to)

This means stuffing 8 trannies onto or into the chassis before I even start with caps, bias pots, sockets etc.

As the front end of the amp is just an input selector and a switched attenuator, I'm thinking that there's barely any difference between what I'm building and a passive pre with two monoblocks.

It'd certainly make for a far neater layout (and I have a couple of spare 2 x 12v AC toroids that I can use to feed both the input tubes and each channel's neg bias supply).

Is there a good reason not to go this way?

[aardvarkash10]

three times the chassis to make!

Upside - you get to fudge up a whole lot of sexy looking handmade interconnects.

Good to hear its all progressing Drew...

[chrish]

Sounds like you have a couple of options.

1. Reduce number of transformers.
2. increase number of chassis.

Why so many transformers?

You may want to consider as an option a separate power supply chassis, and then a single amp chassis with the passive switching and volume control.

[Eli Duttman]

I like Chris' suggestion, as it deals with the interconnect cable issue associated with passive control centers. By definition, a passive control center has a high O/P impedance and poor drive capability. That forces the use of short, low capacitance interconnect cables between the control center and power amp. Mounting the control center in the power amp chassis, an "integrated amp", bypasses the interconnect cable problem.

[tinitus]

how about integrated mono amps ?

[fdegrove]

Hi,

Integrated monoblocks aren't very practical for setting the volume.

Cheers,

[DrewP]

I had pondered Chris's option of a separate PSU too, but with an under 5 in the house, the idea of a HT umbilical concerns me. Far safer to "umbilical" a line level signal. Will not be a long cable run.

As for why so many transformers, it's simply what I had available to me. To be sure, a single mains with B+, a neg bias supply and enough 6.3v AC to run everything would take way less space, but twin B+ trannies without heater or bias is what I've got here.

[chrish]

Regarding HT umbilical, in my opinion, it is probably safer than any valves on display, that is, if the valves are not completely enclosed in the chassis. A child could hit the valve, break the glass and be exposed the the B+ on the plates. If you use a HT umbilical, check out Building Valve Amplifiers by Morgan Jones if you have it, he addresses some of the safety issues. For instance, make sure that the HT end of any connector is the female component so that no fingers can touch HT if disconnected.

Not sure how much space it saves, but the bias can be tapped from the B+ transformer, saving at least that transformer. See how they do it on guitar amps http://www.ax84.com/static/corepower..._Schematic.pdf

Maybe if you post your intended design/schematic and the specifications of your available components someone here might be able to offer some other advice.

Good luck with the project!

Chris

[tinitus]

Originally Posted by fdegrove

Hi,

Integrated monoblocks aren't very practical for setting the volume.

Cheers,

once correct balance is found it stays there but hey, isnt it about very enthusiastic amps

[DrewP]

Will have to draw up schematic and scan. May take a few days. Thanks in advance for assistance.