PX25 DRD Power amp.

[Steve Cresswell]

Here's a nice little DRD amp I've been developing over the past
few months or so.
It started as a Loftin-White and developed into this super-simple
amp. Excluding the PSU, there are only six components per channel.

The PX25 is a TJ meshplate version of the valve as there's no way
I could afford a real one The in line battery grid bias scheme
is the best sounding of the three I tried, (cathode battery,grid battery with input coupling cap and the one you see on the schematic).

Having built a number of cap coupled SE amps and a couple of push-pulls, this
is by far the best sounding amp I've built so far, so much so that I cannot see
myself changing it for quite a while.

Schematic below



The amp is a breadboard at the moment and is using an E182CC in place of the 5687 for slightly more gain but will be boxed over the next few weeks.



Steve.

[dowdylama]

Nice job - my only experience is with the KR PX25s; but it's a wonderful sound!

[jane]

Is B+ delayed? Before the driver tube warms up the PX25 will not be biased.

Jan E.

[Steve Cresswell]

The amp starts up slowly courtesy of the GZ37 rectifier. It can just be seen poking up behind the two big PSU caps. It takes 15 seconds before the B+ appears, by which time the driver has warmed up.

The amp powers up silently with no distress. I learned the ropes in terms of direct- coupled amps with a Loftin-White 2A3 amp I built about four years ago. That was a good amp, but nowhere near the sound quality of this one, primarily I suspect due to the use of 12AX7s as input valves, which with hindsight, probably had insufficient current to drive the 2A3s without slew-rate limiting becoming evident.

I got sidetracked into more complex amps and have with this amp, made a welcome return to the delights of ultra-simple, single-ended circuits

I'm about to change the AAA cells for a single 3V lithium coin battery. That should last for years before needing replacement.

Steve

[Steve Cresswell]

Hi

I'm currently using Hammond chokes as plate loads for the driver valves.
I'm looking to improve these.

Does anyone have any recommendations for good chokes?

Steve

[ystkkmkr]

What battery did you try for cathode battery bias?

My ULTRAPATHed free-lunch 2A3 monos are essentially very similar to yours, so I am very interested in your recent experiments and future plans.

Thanks,
Yoshi

[Steve Cresswell]

Hi Yoshi,

I used a Nokia rechargeable 3.4V cellphone battery in the cathode. You need rechargeables if doing cathode battery bias and they need to be able to withstand the constant bias current trickle charge or they can explode or overheat.

With cathode battery bias, there was great bass but the life in the mids and top seemed to be constrained. I know this is at odds with what Steve Bench found and what several other people have experienced but I didn't like it in my amp.

Steve

[dsavitsk]

Nice design.

Originally posted by Steve Cresswell
I'm currently using Hammond chokes as plate loads for the driver valves.
I'm looking to improve these.

Does anyone have any recommendations for good chokes?

I'd say Electra-Print is a good place to start, or Magnequest, Intact, or K&K (Lundahl) should also have chokes that will work.

[Steve Cresswell]

Thanks for the choke recommendations. I'll do my homework with these companies products before deciding which chokes to go for.

As to future plans I might try out LED bias on the drivers just for the sake of completeness. My only concern with this is that the impedance of these things varies with frequency, which under signal conditions is bound to introduce distortion. Whether this is of any consequence or not I'll have to find out by trying it. But IME, after the power supply, it is often the quality of the driver that is the biggest influence on the sound of an amp. The old ,"garbage in, garbage out" rule.

So the performance of the driver has to be beyond reproach for the magic to appear.

Steve

[kevinkr]

Originally posted by Steve Cresswell
Thanks for the choke recommendations. I'll do my homework with these companies products before deciding which chokes to go for.

As to future plans I might try out LED bias on the drivers just for the sake of completeness. My only concern with this is that the impedance of these things varies with frequency, which under signal conditions is bound to introduce distortion. Whether this is of any consequence or not I'll have to find out by trying it. But IME, after the power supply, it is often the quality of the driver that is the biggest influence on the sound of an amp. The old ,"garbage in, garbage out" rule.

So the performance of the driver has to be beyond reproach for the magic to appear.

Steve

That is also the case with batteries, and depending on the battery it can be significantly worse, particularly above a couple of kHz. The led bias scheme has worked well for me with a somewhat similar design using 5842 to drive 71A.

I have experimented with a number of different types of batteries in the cathode location and have found a great divergence in results depending on the chemistry. I have never found a NiMH that I could stand to listen to, but some of the Sanyo NiCads sounded very good. You need to size the cell for trickle charge application at the current found in your cathode circuit. And never EVER use an alkaline in the cathode circuit - one of my friends did so despite my warning and had oozing, bulging cells in minutes..

Your current AAA cells will last their shelf life in your application, but I would try a small film (or not so small film cap) in parallel to see what effect it has. The lithium cells I tried actually did not sound that wonderful, but that might be the particular ones I used. (Radio Shack) In theory the lithium could/should be better, but if you like it the way it is there really is no problem with just leaving as is.

Being in the UK I should think you would want to talk to Sowter about audio chokes, shipping from the USA in particular could be a little costly.