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.
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
An externally hosted image should be here but it was not working when we last tested it.
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.
An externally hosted image should be here but it was not working when we last tested it.
Steve.
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
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
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
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
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
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
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
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
Nice design.
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 said:
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.
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
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
Steve Cresswell said:
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.
I'll agree with Kevin that LEDs can work really well.
I didn't notice where you were. Agreed that Sowter is worth looking into, but the dollar may be weak enough that the American made stuff might be worth shipping. A pair of chokes is not that heavy.
kevinkr said:
I didn't notice where you were. Agreed that Sowter is worth looking into, but the dollar may be weak enough that the American made stuff might be worth shipping. A pair of chokes is not that heavy.
Thanks guys.
The Sowter option might be the one to go for.
I was thinking of bypassing the battery with a small cap to see what if anything happened to the sound. The AAA alkaline cells sound excellent by themselves as in- line bias batteries, so whether a cap could improve things we can only wait and see.
I tried a 3V lithium cell in the grid cct but although superficially, there did not appear to be much of a difference in sound quality, there was something not quite right about the presentation that I could not pin down. Reverting to the alkaline cells removed that feeling and restored peace of mind: strange.
I see Kevin, that you too did not have a positive outcome when using lithium cells in the cathode position.
Steve
The Sowter option might be the one to go for.
I was thinking of bypassing the battery with a small cap to see what if anything happened to the sound. The AAA alkaline cells sound excellent by themselves as in- line bias batteries, so whether a cap could improve things we can only wait and see.
I tried a 3V lithium cell in the grid cct but although superficially, there did not appear to be much of a difference in sound quality, there was something not quite right about the presentation that I could not pin down. Reverting to the alkaline cells removed that feeling and restored peace of mind: strange.
I see Kevin, that you too did not have a positive outcome when using lithium cells in the cathode position.
Steve
Steve Cresswell said:
Hi Steve,
Actually did not use lithium in cathode position but in same configuration you tried with AAA and Li button cell - did not like the result.
Late last night I changed the bias scheme from battery grid to LED cathode bias.
I must say those LEDs looked very pretty, glowing through the perspex plate that everything is mounted to.
On the sound quality front it was slightly better than resistor cathode bias and in isolation I might have left it at that: if I had not heard battery grid bias first.
Cathode LED bias gave a civilised presentation but in comparison to the grid bias it was lacking air and sparkle at the top and grunt at the bottom.
Simply put, the in-line battery grid bias scheme gave a full, detailed and dynamic quality to the presentation, with which the LED bias simply could not compete.
The noise floor was also quite a bit lower than with either resistor or LED cathode bias, revealing more of the microdynamic qualities that give music that live feel. A bit of folk in the shape of the "Kieran Goss Trio Live" album showed this off to startling effect.
With the LED bias, it was a pleasant experience, Goss's warm tones and acoustic guitar playing laid out in front of the soundstage-lovely.
With the battery bias, Gareth Hughes double bass playing was brought out and Annie Kinsella's backing vocals were far better defined. Audience noises were better separated at the back of the soundstage and the whole atmosphere of the live performance was portrayed with much greater authority than with the LED bias.
The old Linn "listen to the tune" ethos was very evident with the battery bias.
It was easy to switch the attention from one instrument to the other and follow the strands separately or just sit back and be engrossed in the performance as a whole.
In this amp battery bias in line with the signal is very successful. I'm going to try a small cap across the battery but I think I've hit the spot with this amp and the breadboard stage is now completed.
I'll post some pics when I've boxed it.
Steve
I must say those LEDs looked very pretty, glowing through the perspex plate that everything is mounted to.
On the sound quality front it was slightly better than resistor cathode bias and in isolation I might have left it at that: if I had not heard battery grid bias first.
Cathode LED bias gave a civilised presentation but in comparison to the grid bias it was lacking air and sparkle at the top and grunt at the bottom.
Simply put, the in-line battery grid bias scheme gave a full, detailed and dynamic quality to the presentation, with which the LED bias simply could not compete.
The noise floor was also quite a bit lower than with either resistor or LED cathode bias, revealing more of the microdynamic qualities that give music that live feel. A bit of folk in the shape of the "Kieran Goss Trio Live" album showed this off to startling effect.
With the LED bias, it was a pleasant experience, Goss's warm tones and acoustic guitar playing laid out in front of the soundstage-lovely.
With the battery bias, Gareth Hughes double bass playing was brought out and Annie Kinsella's backing vocals were far better defined. Audience noises were better separated at the back of the soundstage and the whole atmosphere of the live performance was portrayed with much greater authority than with the LED bias.
The old Linn "listen to the tune" ethos was very evident with the battery bias.
It was easy to switch the attention from one instrument to the other and follow the strands separately or just sit back and be engrossed in the performance as a whole.
In this amp battery bias in line with the signal is very successful. I'm going to try a small cap across the battery but I think I've hit the spot with this amp and the breadboard stage is now completed.
I'll post some pics when I've boxed it.
Steve
Sounds good, that is what the breadboarding phase is all about.. 😀
Can't wait to see the pix of the final product. What do you think of those TJ PX25 so far?
Can't wait to see the pix of the final product. What do you think of those TJ PX25 so far?
Kevin ... or anyone else that might know, any experience in differences made w/r/t/ the amount of battery grid bias. That is, I am working on an amp that I was planning to bias in a similar way, but I need ~18V of bias. I figured I'd just use a pair of 9V batteries in series, but I didn't know if this would have more adverse influence than the small 3V bias used here. Any thoughts?
dsavitsk said:
Hi Dave,
This could well be an issue, in this case I would still use a pair of 9V batteries but in a more conventional scheme which would require the use of a coupling cap and resistor to inject the bias. The internal impedance of a 9V battery is about an order of magnitude higher (or more) than a single AAA battery. I have not tried it for this reason..
kevinkr said:
Doug 🙂
kevinkr said:
Okay. I am actually looking to do it a little differently with the batteries not quite as in the signal path and with an input transformer (in place of the input cap). I think this takes care of the impedance issue. Here's the schematic:
An externally hosted image should be here but it was not working when we last tested it.
Hi Doug,
I think that will work pretty well as drawn, and worst case you can always add a small cap across the batteries to improve high frequency performance.
Sorry about the "Dave" thing, I was thinking Doug, and then wrote Dave, but then again I have done that before as you well know.. Ah, the joys of pre-senescence 🙄
I think that will work pretty well as drawn, and worst case you can always add a small cap across the batteries to improve high frequency performance.
Sorry about the "Dave" thing, I was thinking Doug, and then wrote Dave, but then again I have done that before as you well know.. Ah, the joys of pre-senescence 🙄
In deviating from 😉 rules, your PSRR is no longer optimal.
You floated cathode and plate of V2 on common PS ripple,
but the plate of V1 and grid of V2 float common to ground.
Without going completely back to LW, this is still fixable.
Simply inject 1/MuV1 ripple back into the cathode of V1.
Amplified by MuV1, Now everything at V2 floats together.
Negative cathode feedback (from V1) and positive loop
feedback (from V2) are equal and cancel.... No feedback!
You floated cathode and plate of V2 on common PS ripple,
but the plate of V1 and grid of V2 float common to ground.
Without going completely back to LW, this is still fixable.
Simply inject 1/MuV1 ripple back into the cathode of V1.
Amplified by MuV1, Now everything at V2 floats together.
Negative cathode feedback (from V1) and positive loop
feedback (from V2) are equal and cancel.... No feedback!
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