I see Q9 and 10 but the other two are Q3 and Q6. Is that right? Where's Q6? 
dinu,
Welcome to the A75, it's not for everyone!
But if you manage to make it, wouahh ...
I was helped by Bob, and many others, but I lost everything
while crashing my HD... Happy to put some papers a side...
And since the A75 there has been the Bosoz, Ono, Aleph-X, and tube SE
Exept for the fan regulation I kept all prints, but managed to loose this one.
Best regards.
Alain.
Welcome to the A75, it's not for everyone!
But if you manage to make it, wouahh ...
I was helped by Bob, and many others, but I lost everything
while crashing my HD... Happy to put some papers a side...
And since the A75 there has been the Bosoz, Ono, Aleph-X, and tube SE
Exept for the fan regulation I kept all prints, but managed to loose this one.
Best regards.
Alain.
No problem. My first build was the zen V7 (induction loaded output) It has three 1KVA transformers in it. It must weigh damn near 90 pounds. I thought THAT was tricky. I'll finish the A 75 if it kills me but I hope it's worth the effort.
I was just talking to Bob about the BOSOZ. It's built but still not working right. I think it's the volume control.
I was just talking to Bob about the BOSOZ. It's built but still not working right. I think it's the volume control.
Morning everyone. The original problem I started the string with is definitely fixed. THANK YOU ALL. The power supply has less ripple than I can measure with the MM, fan runs nice and quiet and the LED works and is not too bright. I put a ferrite band around the transformer which also seems to suppress any stray magnetic flux. Front end goes in tonight. I'm trying to decide if I need bother with heat sinks on the front end 220's. The board is right in front of the fan which is moving a decent amount of air past it. I'm guessing it should keep them pretty cool although there's no such thing as too much heat sinking.
What's the consensus on R81? I was thinking of using a 3 position switch. (ON, 500ohm, and off) That should give me (full, 50%, and 100%) feedback. I'd love to hear what you all found sounds best?
What's the consensus on R81? I was thinking of using a 3 position switch. (ON, 500ohm, and off) That should give me (full, 50%, and 100%) feedback. I'd love to hear what you all found sounds best?
Hello Bob,
Comes up all your advice seems just replace IRF610 by Vbe multiplier and uses 20 turns VR is very simple.
Yesterday in my bench found two N Channel MOSFET 2sk216. Its characteristics is similiar to output stage K176. I think it can help to improve the bias voltage stablity.
How to modify the voltage multipler resistor network ?
Thanks too much
CK
Comes up all your advice seems just replace IRF610 by Vbe multiplier and uses 20 turns VR is very simple.
Yesterday in my bench found two N Channel MOSFET 2sk216. Its characteristics is similiar to output stage K176. I think it can help to improve the bias voltage stablity.
How to modify the voltage multipler resistor network ?
Thanks too much
CK
CK,
I'm sure someone could calculate ideal resistor values, but since like the original your Vgs transistor is similar to your output devices, I'd try the circuit as is. Just swap in your 2sk216 and see how it works with no output stage connected.
If you cannot adjust the bias voltage low enough to almost cut off your output stage, I'd use a smaller R80 as NP recommends in the article. Being able to get up to 4 volts ought to be plenty, although I have not worked with lateral fets. I suggest looking for an amp design that uses them and see what bias voltage is used.
I'm sure someone could calculate ideal resistor values, but since like the original your Vgs transistor is similar to your output devices, I'd try the circuit as is. Just swap in your 2sk216 and see how it works with no output stage connected.
If you cannot adjust the bias voltage low enough to almost cut off your output stage, I'd use a smaller R80 as NP recommends in the article. Being able to get up to 4 volts ought to be plenty, although I have not worked with lateral fets. I suggest looking for an amp design that uses them and see what bias voltage is used.
Jeez, I hate the search engine here. It's bad enough that I lose threads, but even knowing what I'm looking for doesn't seem to help.
Anyway, life being what it is, I had cause to dig out a Hafler DH-200 schematic the other day on a service-related sort of thing and Bob's right--it's a bipolar bias circuit. In fact, the whole front end is bipolar, which isn't at all what I had remembered. I thought the front end differential was bipolar and everything else was MOSFET from there on. Oh, well, point to Bob. Thanks, man.
Grey
Anyway, life being what it is, I had cause to dig out a Hafler DH-200 schematic the other day on a service-related sort of thing and Bob's right--it's a bipolar bias circuit. In fact, the whole front end is bipolar, which isn't at all what I had remembered. I thought the front end differential was bipolar and everything else was MOSFET from there on. Oh, well, point to Bob. Thanks, man.
Grey
dinu said:
dinu,
Your "switch" idea sounds like a good one.
You are unlikely to get a consensus on R81: the A75 sounds pretty darn good in all of its configurations. The flexibility offered by the design (e.g., percentage of global feedback, folding cascode) allows you to voice your end-to-end system sound in accordance with your personal taste and listening habits.
In my case, with 50% global feedback and stock configuration (no folded cascode), the sound is detailed, neither bright nor dark.
With 0% global feedback and folded cascode, the sound is darker - as in less bright. It is also more "laid back" - details don't draw as much attention, the image is recessed a feet or two, the whole spectrum is seamlessly integrated. The result is very pleasant. For now I am sticking with this... until I move on!
Pierre
PS: I would be curious to hear the opinion of others!
Source: Sony DVP-S9000ES
Preamp: passive or BOSOZ
Amp: A75 with reduced bias (1 Amp per side)
Speakers: DIY MTM towers (PHL 1340 and Aurum Cantus G2)
Music: Classic rock, jazz (trio), classical piano, and opera.
Thanks Pierre. I haven't had a chance in a good three weeks now to even touch it. Did you try 100% feedback? Am I seeing a pattern? 0% = less pronounced detail, more reserved sound stage. 50% feedback = slightly brighter sound with a bit more forward soundstage, Can I take a leap and assume what 100% will yield? Possibly very bright detail and sound staging further forward in the room? I too would love to read more oppinions.
PS
I previously asked about output protection. I ordered and received the Velleman kit from Audioxpress. Have yet to assemble, and now that I have it, I realized I have very little room for it. Typical !
PS
I previously asked about output protection. I ordered and received the Velleman kit from Audioxpress. Have yet to assemble, and now that I have it, I realized I have very little room for it. Typical !
A75 sound and feedback
As requested, here are some personal observations pertaining to the A75 sound using folded cascode and a balanced line stage. Beware that all this is very subjective. For the following, I listened to Bill Evans (Analog Production SACD), Angela Hewitt (Hyperion CD), Artur Schnabel (NAXOS CD), Eagles (DCC CD), and Diana Krall (Verve SACD).
With 100% global feedback: detail, precision and holographic spatial cues convey an impression of listening "into" the music. On Diana Krall, I notice a hint of sibilance; on other albums, a crisp top end. Good rythm and excitement.
With 0% global feedaback: the top end is smooth, the sound stage wide open, and the overall listening experience relaxing. A word that springs to mind is "natural".
Both settings sound nice on their own right. Please note that the differences discussed here are subtle, and that I find them to be challenging to describe. Upon casual listening, one might find that the two settings sound very close.
If you build and listen, please share your findings!
Pierre
As requested, here are some personal observations pertaining to the A75 sound using folded cascode and a balanced line stage. Beware that all this is very subjective. For the following, I listened to Bill Evans (Analog Production SACD), Angela Hewitt (Hyperion CD), Artur Schnabel (NAXOS CD), Eagles (DCC CD), and Diana Krall (Verve SACD).
With 100% global feedback: detail, precision and holographic spatial cues convey an impression of listening "into" the music. On Diana Krall, I notice a hint of sibilance; on other albums, a crisp top end. Good rythm and excitement.
With 0% global feedaback: the top end is smooth, the sound stage wide open, and the overall listening experience relaxing. A word that springs to mind is "natural".
Both settings sound nice on their own right. Please note that the differences discussed here are subtle, and that I find them to be challenging to describe. Upon casual listening, one might find that the two settings sound very close.
If you build and listen, please share your findings!
Pierre
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