A few pre build questions

[epistaxsis]

Evenin all,

Having built a Foreplay2 many years ago I have decided to do the job properly this time!

I have now received my Extended Foreplay3 kit which will be combined with Steve Bench's valve active crossovers - link here: http://greygum.net/sbench/sbench101/#Electronic

As a consequence I have a few questions for the experts hereabouts:

1) Valve rectification:

My phonostage and power amps all use this - is it worth doing?

If so a few pointers in the right direction would be appreciated.

2) Inverted Phase:

What are the implications here?

Do I need to start to swap things around?

3) PSU capacity:

The crossovers use a 6922/E88CC per channel.

I note the post about increasing the current of the heater supply to cope - but what about the HT supply?

3) Hum reduction:

I have  noticed that one of the design considerations for the Foreplay3 was to improve performance in this area with regard to the Foreplay2.

As I will be changing the orientation of the circuitry to cope with relay switched inputs, AV passthrough, the crossovers and a proper stereo ladder stepped attenuator - what do I need to watch out for?

I am intending to keep power and signal wiring *very* separated! <-- to the point that I am considering wiring every ground connection separately...

I look forward to your responses.

[Paul Birkeland]

1) Valve rectification:

2) Inverted Phase:

3) PSU capacity:


3) Hum reduction:

1)  You could indeed use a tube rectifier with the extended FP-III, but you will need a separate filament transformer for that tube (the PT-3 doesn't have sufficient heater current.

2)  If you are worried about phase inversion, you can experiment on the speaker end of the system.  IMO, there is a discernible difference that I can notice at the onset of transients if the absolute phase is off by 180 degrees.

3)  I don't think there is enough PSU capacity to run those PC boards from a PT-3 with DC heaters.  (I do have experience with them, as I used them with my old Straight 8's as an active x-over).  Fortunately, you would be absolutely spot on if you used a Seduction kit with these.  You would have the heater current and B+ current that you would need, the chassis would probably work with the addition of a couple extra holes for the output jacks.

4) Stuffing all of that into an FP-III chassis, you may be able to get away with it, but it is likely that you will end up having to put some critical components in magnetically noisy areas.  Which attenuator/input selector were you considering using?  AV-pass through isn't a huge problem (you could feed the signal into the extra output for this), but powering another PCB under the chassis could be an issue.

-PB

[Paul Joppa]

Just a note - the Extended FP-III uses two independent full-wave bridge rectifiers, i.e. 8 diodes. Four 6X4 or similar tubes would substitute if you can find enough heater power, but you may not have enough compliance voltage for the shunt regulator to work properly.

[epistaxsis]

1)  You could indeed use a tube rectifier with the extended FP-III, but you will need a separate filament transformer for that tube (the PT-3 doesn't have sufficient heater current.

That did cross my mind after I posted.

2)  If you are worried about phase inversion, you can experiment on the speaker end of the system.  IMO, there is a discernible difference that I can notice at the onset of transients if the absolute phase is off by 180 degrees.

OK then I will use the dreaded homo sapiens sapiens audio tool then.

3)  I don't think there is enough PSU capacity to run those PC boards from a PT-3 with DC heaters.  (I do have experience with them, as I used them with my old Straight 8's as an active x-over).  Fortunately, you would be absolutely spot on if you used a Seduction kit with these.  You would have the heater current and B+ current that you would need, the chassis would probably work with the addition of a couple extra holes for the output jacks.

Only problem is that I need a 230V transformer - I had thought about using  a second extended foreplay one to power the crossovers.

I shall have to reconsider this.

Could you pm me or email me about the crossovers - I could do with finding out more. <-- as this isn't really bottlehead stuff I don't want to annoy anyone!

4) Stuffing all of that into an FP-III chassis, you may be able to get away with it, but it is likely that you will end up having to put some critical components in magnetically noisy areas.  Which attenuator/input selector were you considering using?  AV-pass through isn't a huge problem (you could feed the signal into the extra output for this), but powering another PCB under the chassis could be an issue.

Sorry - I didn't make myself clear.

I have my own chassis to use which has plenty of space so I can start moving things around for best effect.

This was why I was asking about what affects what.

The stepped attenuator will be a full stereo ladder probably using esoteric japanese resistors (pre-made this time - I did a dual mono one for my Foreplay2 and even after this many years soldering 96 resistors has somehow lost its appeal...).

After all there are two major upgrades that an be made to a valve amp - the bit at the beginning (the volume control) and the bit at the end (the output capacitor)!!!

The input switch will switch a set of relays to select the input for those sources needing the pre-amp and switching the input to the crossovers between the output of the foreplay and the output of my AV pre-amp as I will be using the main stereo speakers for both (there is no way I am going to pollute my 12" loveliness with that!)

Thankyou for your response.

[Paul Birkeland]

When you order your Foreplay, go ahead and ask for the 240 volt PT-3, and see if the Seduction transformer comes in a 240 volt version.  Both transformers will radiate a whole lot less magnetic radiation than a standard 240v Hammond transformer or similar offering.  Since Bottlehead doesn't sell crossovers (yet), and you are packaging them together with a Foreplay, it doesn't seem super unreasonable to post any questions you have here. 

It is somewhat unfortunate that the PT-3 is not for sale by itself.  You could simply implement another extended FP-III power supply for the crossovers, using a 3rd 6DJ8 as the shunt regulator. 

PJ is also quite correct about the voltage compliance.  If the tube rectifier drops too much voltage, the raw B+ and the regulated B+ will be too close together for the top C4S to work properly.  One can get around that by either lowering the regulated B+ voltage (and reworking the preamp tube operating points), using the stock solid state rectifiers, or maybe doing a tube doubler (difficult).  In my previous preamp builds, solid state rectification has always been a winner!

[epistaxsis]

After much prevarication I have ordered the key parts for another extended foreplay power supply to power up the active crossovers.

Now all I have to do is to work out how to attenuate the HF crossover by 3.5Db....