Hi Steen,
very nice Amps! What you say about the sound is what I hear too but then I´m also biased a bit......I tried them single ended but although OK the balanced input was better even when using inferior cables.
Just a few questions:
What transformers do you use and do you have any mechanical hum/buzz?
Are the inductors with or without a core?
What bias did you go for?
I haven´t done much diy-audio lately as I tried putting a new (diy) ecu in my car wich took quite some time. But since it´s gone into wintersleep I will finally try the dc servo for absolute dc offset in the next few weeks.
William
very nice Amps! What you say about the sound is what I hear too but then I´m also biased a bit......I tried them single ended but although OK the balanced input was better even when using inferior cables.
Just a few questions:
What transformers do you use and do you have any mechanical hum/buzz?
Are the inductors with or without a core?
What bias did you go for?
I haven´t done much diy-audio lately as I tried putting a new (diy) ecu in my car wich took quite some time. But since it´s gone into wintersleep I will finally try the dc servo for absolute dc offset in the next few weeks.
William
Thanks tenderland, I am sure yours will look great if you really want them to. Persistance pays off in the end
William; thanks
The trafos are 1000VA 2x18V's. There is a very slight hum, like in, I have to put my ear next to the amp to hear it. (With no music) So its really only a little. In the speakers there is none, whatsoever.The inductors are aircored (no core!) handwound by my friend Magura. The amps are biased for 6 amps right now, which I will increase when I make a few mods like putting in foil resistors in the feedback loop and decreasing the CCS gain. For now the CCS contributes with approx 1/3 of the total output. I have the idea that the amp will sound better with a lower CCS contribution, since my speakers doesnt get near 4ohm's impedance. Because the heatsinks get at the max 44 Deg C right above the fets, I think the amp can take some more bias current. I will keep the heatsinks at 50 deg C at the most though
I remember you were talking about a DC servo in GL's thread. Looking forward to hear about your results. My A-X's acually behaves very exemplary, regarding DC offset. At the speakerterminals the offset is at all times less than 20mV. At startup the absolute DC offset is about 1,5 Volts, drifting to 0V after 1/2 an hour of warmup. It drifts slightly with temp. variations but not a lot. It swings around 0V when warm.
I had some of the DIYaudio members dropping by today, which was very nice
I even think they liked the sound of the A-X's I cant resist posting one more pic of Maguras creations. (I didnt ask
) They make me think of a UK Spitfire with Rolls Royce engine Well, pretty impresive work and, very very well made Wish I was a Wizzard with CNC tools It would take a while to walk you through the details which are as impressive as they look, so I will let the pic speak for it self BTW the top and bottomplate is machined Carbon fibre, I guess to save weight which will be around 100 kg per monoblock.Steen
Edit, William this is the inductors. ( I realise this thread is growing a bit long
)http://www.diyaudio.com/forums/showthread.php?postid=1095033#post1095033
Attachments
steenoe said:
I had some of the DIYaudio members dropping by today, which was very nice
I cant resist posting one more pic of Maguras creations. (I didnt ask
Steen
Yes the AX worked very well with Steen proac clones, best sound so far at Steens place
Latest pics of my little amps can be found here http://www.briangt.com/gallery/album40
Magura
Hello steen, thanks for a nice listning session yesterday, i really enjoyed it.
Well, what thrilled me most, was the sound from your'e old vinyl's, I never heard anything like that comming from a pickup before ! Damn... i got plenty of projects comming, but i simply have to make a Riaa preamp now !!...
The sound from your'e Aleph's, are really amazing (eventhroug it was music i was not used to), the soundstage is wide open, and it's like every single detail is comming through. Excelent job Steen.
I realise, that you and Magura, are a giant resource, not only for me, but also for the hole forum here, thank you guy's.
At last, before any would start whinning, i put a link so that you can look at my media-pc.
You can find some pictures
Here
Jesper.
Well, what thrilled me most, was the sound from your'e old vinyl's, I never heard anything like that comming from a pickup before ! Damn... i got plenty of projects comming, but i simply have to make a Riaa preamp now !!
...The sound from your'e Aleph's, are really amazing (eventhroug it was music i was not used to), the soundstage is wide open, and it's like every single detail is comming through. Excelent job Steen.
I realise, that you and Magura, are a giant resource, not only for me, but also for the hole forum here, thank you guy's.
At last, before any would start whinning, i put a link so that you can look at my media-pc.
You can find some pictures
Here
Jesper.
Absolute DC
William,
It is my own experience that if all (16) Power FETs in one channel are closely matched, and that they are thermally well coupled to each other (to within 1 degC transistor case temperature at full bias), then absolute DC is no real issue.
I have 100 ohm resistors at the ouputs to ground, and 10k DC feedback resistors to the diff pair source, and I get absolute DC from 2V at 20 degC to 10mV at 60 degC after 4 hours; less than 0.5V within an hour. Diff DC is less than 2mV at all times (well, we have feedback for diff DC).
(Supply voltage +/-24V, bias 7.7A total, per channel)
Patrick
William,
It is my own experience that if all (16) Power FETs in one channel are closely matched, and that they are thermally well coupled to each other (to within 1 degC transistor case temperature at full bias), then absolute DC is no real issue.
I have 100 ohm resistors at the ouputs to ground, and 10k DC feedback resistors to the diff pair source, and I get absolute DC from 2V at 20 degC to 10mV at 60 degC after 4 hours; less than 0.5V within an hour. Diff DC is less than 2mV at all times (well, we have feedback for diff DC).
(Supply voltage +/-24V, bias 7.7A total, per channel)
Patrick
Hi Patrick,
it is possible that the temperature distribution is less then ideal in my case. The front panel is plexi, the rear panel is aluminium. I start at around 5 volts at 20° going down fast. This behaviour is the same for both amps. (80 ohms to ground, 10k dc-feedback resistors).
There are other influences though like the thermal behaviour of the current source.
With a dc servo I also hope to reduce the mechanical transformer hum by reducing the assymmetrical load.
William
it is possible that the temperature distribution is less then ideal in my case. The front panel is plexi, the rear panel is aluminium. I start at around 5 volts at 20° going down fast. This behaviour is the same for both amps. (80 ohms to ground, 10k dc-feedback resistors).
There are other influences though like the thermal behaviour of the current source.
With a dc servo I also hope to reduce the mechanical transformer hum by reducing the assymmetrical load.
William
William, they are 4,7k. The single most important thing that made my amps behave, was when I raised the source resistor value. Initially I had 0,47r. With that value the amp was impossible to control, with either the Absolute DC or the bias going crazy. I raised the value to 0,8r and everything was much better. I changed some resistor values in the bias adjustment to be able to get 1A through each fet. The fets are matched in sets of 6 matches, using 2 such sets per channel.
You are welcome
Yep, it was pretty nice. Vinyl surprises many people, they seems to have forgotten that they sound better than CD's Especially when its going through a decent RIAA preamp (read the Pearl).Thats a nice Media-PC project you have there. A thing like that is supposed to sound better than a CD-player, so I might try and make something like that.
Steen
BTW Patrick, did you post pictures of your A-X's?
William,
5V is not bad. I would not bother too much if I were you.
The MPSA18s (or ZT450s) is another cause of DC drifts as they have different thermal behaviour to the diff pair, depending on what you use (I use JFETs).
But do give it a try with servos, although I doubt whether it would cure your tranformer problem. 5V / 80 ohm is 63mA common mode current; if that alone causes humm in your (1000VA?) transformer !!!!!!
We had 3 transformers from a small German supplier who only do business with companies, and we never had any problems with humm. You can only feel a tinny bit of vibration with your finger, but you can hear absolutely nothing when it fits in a rigid housing even when you placed your ears ON it.
Contact me by private email if you need more info.
And Steen no I did not post my pictures. I once posted a few of my preamp and some people tried to make jokes on them, like they did to everyone else. Well I do not need that, and I know what I have built and why I built them like that. So no pictures in the public domain. I shall send some to Nelson eventually as a token of thanks to him personally, and to a few friends on the DIY forum with whom I correspond in private.
Regards,
Patrick
5V is not bad. I would not bother too much if I were you.
The MPSA18s (or ZT450s) is another cause of DC drifts as they have different thermal behaviour to the diff pair, depending on what you use (I use JFETs).
But do give it a try with servos, although I doubt whether it would cure your tranformer problem. 5V / 80 ohm is 63mA common mode current; if that alone causes humm in your (1000VA?) transformer !!!!!!
We had 3 transformers from a small German supplier who only do business with companies, and we never had any problems with humm. You can only feel a tinny bit of vibration with your finger, but you can hear absolutely nothing when it fits in a rigid housing even when you placed your ears ON it.
Contact me by private email if you need more info.
And Steen no I did not post my pictures. I once posted a few of my preamp and some people tried to make jokes on them, like they did to everyone else. Well I do not need that, and I know what I have built and why I built them like that. So no pictures in the public domain. I shall send some to Nelson eventually as a token of thanks to him personally, and to a few friends on the DIY forum with whom I correspond in private.
Regards,
Patrick
Fair enough, your call. Just sad to read that a few members does something like that. It was probably some of the socalled "experts" that never build a thing, themselfes. Strange attitude, by the way, that these members have; what would this forum be if noone dared to post a project, because they were afraid of being mobbed?
Steen
EUVL said:
Patrick,
be gratefull they did not start commenting on what you did wrong and how it should be done.
By gents who construct in theory only, that is the true joke.
The fun part of this club that you can separate the ones who can teach you something from the BS-ters within a few months, if only the real world would be like that.
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