McCurdy AU300 clone - development thread

[amplexus]

gapped push-pull opt?

My bad- the 2291ungapped. I have a couple builds on the go- one uses the 2290. Tho it should still work fine with either I believe.

 

[iangomes]

Wow, very cool! I have 8 with the JS transformers in them and love them, Never thought I would ever see a proper clone, though!

 

[JMan]

Every once in awhile I check in on this thread to look at those amazing pictures again. Curious to know if there's been any further thought about making this available as a project for the forum...

 

[lightningbefore]

Curious how much this kit costs all in with PSU? Is there a build guide that comes with it? Looks really fun.

 

[kaltavilla]

Any progress on making the kit available?

 

[amplexus]

Curious how much this kit costs all in with PSU? Is there a build guide that comes with it? Looks really fun.

No idea- it'll depend on what I land on for PCB features, and how much interest there is- ie: how many boards I'll order from fab etc.. This got seriously backburnered this year because of a bunch of big projects/contracts that happened. But I think I can put it back on the todo list!

 

[JMan]

I'm still interested, if/when it ends up happening!

 

[punk]

i'm definitely interested. i'll probably be interested in 8 pcb's, depending on price. thanks for your hard work on this so far.

 

[ostrol]

Bumping, interested in this! Would love to know the cost of building one of these

 

[Ilya]

Is there any info on the input and output transformers turns ratio?
I'd guess something like 1:7 on the input and 4:1 on the output? Anybody?

 

[My3gger]

Is there any info on the input and output transformers turns ratio?
I'd guess something like 1:7 on the input and 4:1 on the output? Anybody?

OP used Cinemag Quad Eight 37,5-150-600:50k (with secondary center tap) input having ratio of at 1:9 wired as 600:50k, 1:18 wired as 150:50k, etc. This is probably close to ratios used in original, typical 1:7 or 1:10 came later. 4:1 push pull output was also common for this type of circuit. I don't have data for originals to say for sure what they were, Amplexus and EMRR should know more.
Input's secondary has very long wires possibly picking interference, high end response should be fine without shield. I like it having no more than a few cm long sec. signal wire (shortest possible) and use relays for switching DI input, or changing load. Searching the forum finds nothing, i might still have lengthy mail from Cinemag about this matter.
Small details like this are important when working with tiny mic signals.

 

[Ilya]

As it is now, the gain of the circuit is rather high even in the minimum setting. I'm wondering if it's reasonable to do the gain adjustment at the first stage as well. For example, set minimum gain at the second stage and arrange the same circuit for gain adjustment for the first stage as it is arranged at the second stage (cathode resistance change). When gain is maxed at the first stage, the second stage gain adjustment comes into play.
Any thoughts/objections?

 

[amplexus]

As it is now, the gain of the circuit is rather high even in the minimum setting. I'm wondering if it's reasonable to do the gain adjustment at the first stage as well. For example, set minimum gain at the second stage and arrange the same circuit for gain adjustment for the first stage as it is arranged at the second stage (cathode resistance change). When gain is maxed at the first stage, the second stage gain adjustment comes into play.
Any thoughts/objections?
View attachment 102020

Could integrate that on a single stepped 2p switch with enough positions, right? Like second stage at minimum until click 6 and then 1st stage stays strapped at max gain and the second stage carries on through the switch travel…

Is that kinda what you’re thinkin?

 

[amplexus]

OP used Cinemag Quad Eight 37,5-150-600:50k input having ratio of at 1:9 wired as 600:50k, 1:18 wired as 150:50k, etc. This is probably close to ratios used in original, typical 1:7 or 1:10 came later. 4:1 push pull output was also common for this type of output. I don't have data for originals to say for sure what they were, Amplexus and EMRR should know more.
Input's secondary has very long wire possibly picking interference, high end is probably fine without shield. I like it having no more than a few cm long sec. signal wire (shortest possible) and use relays for switching DI input, or changing load. Searching the forum finds nothing, i might still have lengthy mail from Cinemag about this matter.
Small details like this are important when working with tiny mic signals.

The reason for having the I/Pt off-board was mostly cus I wasn’t sure which of the two or three transformers I was
Gonna land on while prototyping. Tho there’s been no issue with noise or interference in this prototype build with the longer leads, the plan has been to do a board mount IPT. Both the cinemag and the carnhill i knocked in worked a treat so since i already have a footprint built for the carnhill and it’s readily available that’ll probably end up being the one.

 

[Ilya]

Could integrate that on a single stepped 2p switch with enough positions, right? Like second stage at minimum until click 6 and then 1st stage stays strapped at max gain and the second stage carries on through the switch travel…

Is that kinda what you’re thinkin?

Exactly! That’s what I thought of doing. You described it in a much more elegant way than I did)
I was thinking about noise due to increased cathode resistance, but I need to refresh my knowledge about tube circuits and noise contribution of various components.

 

[amplexus]

Exactly! That’s what I thought of doing. You described it in a much more elegant way than I did)
I was thinking about noise due to increased cathode resistance, but I need to refresh my knowledge about tube circuits and noise contribution of various components.

I Could run the numbers- but it’s easy enough to knock into the prototype board i’ve already got built! Things are slow in the next couple weeks so I can maybe put this on the front burner again.

 

[amplexus]

For anyone who might be interested, would you prefer:

1. Both transformers off-board (flying leads for both, allows more transformer options, but less compact)
2. Output Transformer off-board, Input transformer on board thru-hole mount (Set up for Carnhill transformer- simpler install more compact)
3. Both transformers on-PCB (Both transformers Carnhill)

The prototype PCB is currently set up for both transformers on flying leads. I am leaning towards option 2 myself.

 

[JMan]

I think option 2 sounds like a nice compromise of space and options, but I'd be happy with any of them if the votes go a different way. FWIW my least favorite is option 3, only because I feel that the physical weight of most output transformers on a pcb adds more considerations regarding adding support.

 

[My3gger]

Did you put pcb connection at one side to ease making of modules? I'm not sure how many people will want kits for more than 1-2ch for reasons already discussed.
Option 1 might be preferable because it allows use of all kinds of transformers some of us already have.
A few changes to the layout might allow making of 2 tube preamps like Langeving AM-5116b, UA 1016, experimenting with new designs ideas like from "PP cross coupled" thread, maybe even RCA BA-21 with some trace cutting, etc. Search didn't show any pcb for PP tube preamps despite being simple and very interesting.

 

[amplexus]

Ok what if i did a footprint for the carnhill- but then brought a parallel set of connections to solder pads at the edge of the card?

Is the general preference for all connections (including gainswitch, i/o, etc…) to be on one card edge?