#26 pre amp

[Rod Coleman]

Originally Posted by Arnulf

I checked the part once again just to make sure: it is a LM350T with F logo and J43 on the right hand side of the logo. I believe this is Farnell item #1417673 although I don't have the receipt anywhere handy. I'm beginning to wonder if this is really a Fairchild part, regardless of what Farnell claims - Fairchild's datasheet specifies tab width to be more than twice of what I've just measured.

If Farnell sent that to me, I'd return it.

It sounds too much like one of the fake semis we hear about.

[Rod Coleman]

If the LT108x needs 5V of headroom to perform properly, we can improve on that.

This circuit uses the same FET in both positions, plus a 3-pin 1,2V reference chip.

If you use the exact parts noted on the drawing, it should work with supplies down to 7V. But you will want to work with real supplies with a tolerance, and some ripple: so 9V +/- 1V should be just right, and allow the use of 10V filter capacitors.

These parts are all very cheap, especially at Rapid:

LM4041
Integrated Circuits from Rapid
STP55NF06L
Rapid Electronics - Electronic Components > Semiconductors

[Rod Coleman]

Bas, This is the known-good sounding 300B heater I have used for the past 5 years.

It only controls current, and has no voltage control. Even so, it still delivers 4.91V, 5 years after its last set-up.

[andyjevans]

Couple of things in this great thread!

With the 300b I fed the LM1084 ccs from a variable bench supply and it seemed to regulate from about 8v. So I reckon on minimum of 3v headroom.

For the power supply box, I use an umbilical a metre long to the signal chassis. It's fixed at the power supply end. I use a Speakon 4 pin for the HT if it's under 350v and a Amphenol AP series 4 pin if it's higher voltage than 400v. I use XLR 4 pin as a rule for the preamp heaters. I put the flament transformer and the diodes in the PSU box, and the 15,000uF cap and the rest of it in the signal chassis. for the HT I put the mains transformer, rectifier, choke and reservoir cap in the PSU box and then a glow tube or actually two glow tubes giving 255v in the signal chassis. So only DC goes to the preamp.

andy

[Bas Horneman]

Bas, This is the known-good sounding 300B heater I have used for the past 5 years

Thanks very much Rod for posting that! Did a search of the fzt1051 and it is surface mount. Do they have through hole stuff? And how well did you have to heatsink those parts?

[kevinkr]

Originally Posted by Rod Coleman

Bas, This is the known-good sounding 300B heater I have used for the past 5 years.

It only controls current, and has no voltage control. Even so, it still delivers 4.91V, 5 years after its last set-up.

Rod, simple and elegant design. I am sure this circuit has much better drop out performance than the typical monolithic, amongst other things. I think I will give this a try in one of my next amplifier projects. Hmm, maybe a respin for a GM70.. Thanks..

[Arnulf]

Originally Posted by Rod Coleman

If Farnell sent that to me, I'd return it.

It sounds too much like one of the fake semis we hear about.

It's not worth the cost of shipping them back.

The regulators work, they just suck as far as cooling is concerned, they simply cannot move the heat away from the silicon fast enough so they keep shutting down. I thought big distributors such as Farnell have their own QC and don't deal with fakes. I've bought numerous components from eBay and never encountered a fake.

[Rod Coleman]

Originally Posted by Bas Horneman

Thanks very much Rod for posting that! Did a search of the fzt1051 and it is surface mount. Do they have through hole stuff? And how well did you have to heatsink those parts?

manufacturer Zetex (Diodes inc) ZTX868 is probably nearest equivalent.

I imagine the BC547C would work OK too.

[revintage]

With the 300b I fed the LM1084 ccs from a variable bench supply and it seemed to regulate from about 8v. So I reckon on minimum of 3v headroom.

Andy,

As mentioned earlier the 1084 regulates with 3V. Still you need ca 5V headroom to get good regulation when used as CCS.

Don´t mind my message to you. I have got the 26´s I need.

[Rod Coleman]

Originally Posted by andyjevans

Couple of things in this great thread!

For the power supply box, I use an umbilical a metre long to the signal chassis. It's fixed at the power supply end. I use a Speakon 4 pin for the HT if it's under 350v and a Amphenol AP series 4 pin if it's higher voltage than 400v. I use XLR 4 pin as a rule for the preamp heaters. I put the flament transformer and the diodes in the PSU box, and the 15,000uF cap and the rest of it in the signal chassis. for the HT I put the mains transformer, rectifier, choke and reservoir cap in the PSU box and then a glow tube or actually two glow tubes giving 255v in the signal chassis. So only DC goes to the preamp.

andy

Andy, I think the umbilical idea is perfect.

Have you tried locating the caps right up close to the transformer? I try to keep the wiring between the rectifier and caps (and the trafo) as short as possible, because the CURRENT running along those wires is a pulse train of 2 to 5A peak, with fast rise time and high-band recovery pulses (even with schottkys!). With long wires there is a risk of electromagnetic coupling into the amp.