DCAudioDIY.com

DC Area Audio DIYer's Community
It is currently May 29th, 2024, 1:10 am

All times are UTC - 5 hours [ DST ]




Post new topic Reply to topic  [ 45 posts ]  Go to page Previous  1, 2, 3, 4, 5  Next
Author Message
 Post subject: Re: 6AH4 preamp
PostPosted: March 11th, 2023, 9:35 am 
Offline
User avatar

Joined: July 24th, 2015, 4:17 pm
Posts: 1709
Location: Parkville, Maryland
Grover Gardner wrote:
David McGown wrote:
Grover,

What are you using for the 470uF cathode bypass capacitor? I have thought about using an LED string to avoid the bypass cap, like 5 red leds (maybe 10 with two parallel strings since 22mA may be a little hot for a single string). I need to build this to try out sometime.

David

If you must use electrolytic caps for CK I would recommend no greater than 100-ufd (the bypass will be good down to 4-Hz.) at a higher WVDC (they will sound better). Some nice 105-degree C low ESR caps like the ones that are available on eBay.

Here is the link: https://www.ebay.com/itm/193721463452?h ... BMytvGjdph

Otherwise, if you have room, film is preferable (polycarbonate) or at least an amalgam of film/electrolytic.

_________________
Walt


Top
 Profile  
 
 Post subject: Re: 6AH4 preamp
PostPosted: March 11th, 2023, 11:36 am 
Offline

Joined: June 4th, 2013, 2:39 pm
Posts: 489
Thanks, David. I'm confused about the recommended capacitance. Does he mean before or after the Maida board? It seems to say that you don't need a cap after the board but maybe I am misunderstanding. I have 5AR4 > 47uF > 30H > 47uF > 30H > 47uF > Maida board > plate resistors. Should I add a cap after the board? Should I reduce the 47uF caps? I feel like it's not set up right to do it's job.

At the moment I have 370vdc at the input of the board and 330 vdc at the output. So the board is dropping 40 vdc. That's enough headroom for the board, yes?

Diodes are a possibility. Right now my preferred output caps, the Fostex, are limited to 250vdc. I hesitate to hit them with higher voltage at turn-on. Will the board absorb the excess voltage? Would I need a delay circuit for the B+?


Top
 Profile  
 
 Post subject: Re: 6AH4 preamp
PostPosted: March 11th, 2023, 12:13 pm 
Offline

Joined: June 4th, 2013, 2:39 pm
Posts: 489
I found a thread on the original design and implemetation of the Maida board and am reading through it now to better understand how it works:

https://www.diyaudio.com/community/thre ... or.209067/


Top
 Profile  
 
 Post subject: Re: 6AH4 preamp
PostPosted: March 11th, 2023, 2:21 pm 
Offline

Joined: June 4th, 2013, 2:39 pm
Posts: 489
Okay, glad I read the original thread. My grounding scheme was wrong, I had it looped, not isolating the input ground from the output ground. So power supply +/- to the input, output +/- to the circuit. Also added 10uF to the ouput of the board. Effectively ripple was being passed along to the common groiund. Dead quiet now with only one choke. Now I'll see how little filtering I can get away with.

I also learned that the board has a soft start up so I'll play around with solid-state rectification. It would be nice to remove the tube recvtifier coloration.


Top
 Profile  
 
 Post subject: Re: 6AH4 preamp
PostPosted: March 11th, 2023, 2:52 pm 
Offline
Site Admin
User avatar

Joined: February 28th, 2013, 10:38 am
Posts: 1697
The best cap for Ck might be no cap at all…. It’s a line stage, you probably don’t need the gain….

Roscoe

_________________
I can explain it to you, but I can’t understand it for you.


Top
 Profile  
 
 Post subject: Re: 6AH4 preamp
PostPosted: March 11th, 2023, 4:15 pm 
Offline

Joined: June 4th, 2013, 2:39 pm
Posts: 489
Roscoe Primrose wrote:
The best cap for Ck might be no cap at all…. It’s a line stage, you probably don’t need the gain….

Roscoe


That's possible, I can certainly try it.

Lots of progress otherwise. With the board properly implemented, I now have

300-0-300 Antek > UF4007 > 100uF > Maida > 10uF

Dead quiet. And I like not fussing with the tube rectifier--it really colors the sound in a way that makes it sort "confusing" as to which type and brand to employ. Swapping 6V6's gives you plenty of options for "tuning." ;-)

The toroid PT definitely affects the Maida board, I have it shifted off the breadboard for now and all is well. I'm thinking of a separate supply with raw DC and filament supply, and put the Maida board on the signal chassis.


Top
 Profile  
 
 Post subject: Re: 6AH4 preamp
PostPosted: March 11th, 2023, 5:03 pm 
Offline
User avatar

Joined: July 24th, 2015, 4:17 pm
Posts: 1709
Location: Parkville, Maryland
Roscoe Primrose wrote:
The best cap for Ck might be no cap at all…. It’s a line stage, you probably don’t need the gain….

Roscoe

I know from my own experience you need at least 15-db gain. My Pass XP-12 originally had 9-db gain and with certain recordings -- either vinyl or CD -- it just wasn't enough to drive my power amps.

I increased the gain to 15-db (lowered the feedback) that worked fine with the secondary effect of improved playback quality.

So -- yes -- if you can get along with gain that is set by the cathode bias resistor -- by all means -- the best capacitor is none at all.

_________________
Walt


Top
 Profile  
 
 Post subject: Re: 6AH4 preamp
PostPosted: March 12th, 2023, 12:02 am 
Offline

Joined: February 28th, 2013, 1:19 pm
Posts: 923
Grover,

Sorry I was out of contact today...was rehearsing all day for a memorial concert this evening. Glad you got everything sorted out on the grounding, I was worried about that. And that the regulator is working as it should.

Antek now sells inexpensive extruded steel toroid covers that should provide some magnetic shielding, not as robust as much more expensive 12GA steel ones they sell, but should help a bit for the sensitivity you are having with the transformer. Worth trying it if you want to retain a compact arrangement, otherwise getting the transformer away is preferable.

Walt, I am not sold on having so much gain in a linestage. I think that it really depends on the overall gain structure of your system and of course the input sensitivity of your amps. I find with Williamson UL PP amps and 90dB speakers one can just get by with a passive with a 2VRMS source, but a small about of gain may help. For me, the best sound is direct from my DAC (Chord DAVE using the digital volume control as a preamp), which can put out 3VRMS in SE mode at full output. Anything else in the way compromises transparency. It makes little sense to attenuate the source by several dB and then add an additional gainstage with the resultant coloration. My Eico HF-60 "clone" amps with a EF86 pentode input tube is so sensitive, I cannot tolerate any additional gain. I have recently been using a 10dB attenuator at the amp input just to get it into the same level as my Williamson amp. Now, my GM70 SET amps probable could use a little help, though the low-ish bias voltage of the triode strapped E280F/7722 input/driver tube I use limits the voltage swing in the negative direction. But I *think* in the case of the PASS (Colburn) circuit, it is more the case of eliminating some of the negative feedback used when you increase the gain as you did. I think you mentioned that in my thread on the Colburn BA2018 linestage.


David

David


Top
 Profile  
 
 Post subject: Re: 6AH4 preamp
PostPosted: March 12th, 2023, 11:19 am 
Offline
User avatar

Joined: July 24th, 2015, 4:17 pm
Posts: 1709
Location: Parkville, Maryland
David McGown wrote:
Grover,

Walt, I am not sold on having so much gain in a linestage. I think that it really depends on the overall gain structure of your system and of course the input sensitivity of your amps. I find with Williamson UL PP amps and 90dB speakers one can just get by with a passive with a 2VRMS source, but a small about of gain may help. For me, the best sound is direct from my DAC (Chord DAVE using the digital volume control as a preamp), which can put out 3VRMS in SE mode at full output. Anything else in the way compromises transparency. It makes little sense to attenuate the source by several dB and then add an additional gainstage with the resultant coloration. My Eico HF-60 "clone" amps with a EF86 pentode input tube is so sensitive, I cannot tolerate any additional gain. I have recently been using a 10dB attenuator at the amp input just to get it into the same level as my Williamson amp. Now, my GM70 SET amps probable could use a little help, though the low-ish bias voltage of the triode strapped E280F/7722 input/driver tube I use limits the voltage swing in the negative direction. But I *think* in the case of the PASS (Colburn) circuit, it is more the case of eliminating some of the negative feedback used when you increase the gain as you did. I think you mentioned that in my thread on the Colburn BA2018 linestage.

David

Everything is a compromise to get good playback. In my case -- the BAT-3Ki could drive my Carys to a level that would have you running from the room screaming in pain. I am running the Carys at reduced gain (the front end tubes I ended up have a mu of 40 as opposed to the original mu of 100) so it will take 2-1/2 volts to drive the Carys to full output. Of course not enough gain was never a problem with my horns until I changed course and hot-rodded the Pass pre-amp. 9-db worth of line-stage gain just did not get the job done with some media. Now a lot depends on the quality of the line stage, and as evidenced at Stuart's get together, no line stage is better if the source can drive the interconnect load and you can get away with it. Yet -- how does one switch between sources? Plus -- vinyl playback needs low noise gain and even with the 500-mv output of my DS Audio kit was not enough through the Pass with some recordings where the cutting level was held back to provide the best possible playback quality (better tracking on playback). Even with CD playback there were a few recordings that did not take advantage of the headroom available provided by that media.

Typically -- the average line-stage has about 20-db of gain with my BAT could providing 26-db if needed.

_________________
Walt


Top
 Profile  
 
 Post subject: Re: 6AH4 preamp
PostPosted: March 12th, 2023, 11:26 am 
Offline

Joined: March 2nd, 2013, 2:43 pm
Posts: 220
Location: Potomac, MD
Speaking of cathode bypass caps, has anyone tried polymer types? I just got some for a power supply application. They are rated for 4 volts and 560 uF, in a package 5/16 inch tall and same in diameter. Seems perfect for a 12AX7. The impressive thing, and is why I got them, is the fantastic ripple-current rating of 6 A. I know that you are not putting 6A through a 12AX7, but to have that rating they have extraordinarily low ESR and ESL. Higher-voltage types are available that could be used for the likes of a 6V6.


Top
 Profile  
 
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 45 posts ]  Go to page Previous  1, 2, 3, 4, 5  Next

All times are UTC - 5 hours [ DST ]


Who is online

Users browsing this forum: No registered users and 5 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
Powered by phpBB® Forum Software © phpBB Group