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Old 15th September 2010, 12:26 PM   #11
damohpi is offline damohpi  Australia
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Apologies for lack of detail, line stage is 50k input. Not sure on configuration yet, as I am still waiting for my order to show up.. Most likely I will start with no buffer stage and onboard BAL/SE.
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Old 15th September 2010, 12:42 PM   #12
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Ok well there is no cap at the output of the BAL/SE stage. So the impedance of your line stage won't really come into play. All you will care about is the impedance of the BAL/SE stage which is ~4.7K. 2uf will give you a corner frequency of 17hz. That would be the absolute lowest I would use. 4.7uf or 10uf would be better. You could even simply keep the 100uf Muse caps and just bypass them with 2uf film(even 1uf in this case). Then almost all the signal will be going through the film cap anyway because they will be in parallel and the signal will take the path of least impedance.
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Last edited by Russ White; 15th September 2010 at 12:45 PM.
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Old 15th September 2010, 02:14 PM   #13
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Originally Posted by BrianDonegan View Post
If you are going to bother changing the output caps, go for some polypropylene films. As far as bipolar electrolytics go, the Nichicon Muse are very very good. Since they haven;t made Blackgates in years, you would likely end up with counterfeits anyway.
Thanks Brian! I'll definitely try the stock caps before spending $$$ on anything exotic. When I was running tubes my faves were Auricaps, but they're gone now as well. [btw, Michael Percy apparently still has the 16v 100uF black gate in stock... hopefully not fakes!]
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Old 15th September 2010, 05:39 PM   #14
avr300 is offline avr300  Denmark
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If you are going to bother changing the output caps, go for some polypropylene films. As far as bipolar electrolytics go, the Nichicon Muse are very very good. Since they haven;t made Blackgates in years, you would likely end up with counterfeits anyway.
Even if they are genuine, they will be old, at best from 2006. (Black Gate (capacitor) - Wikipedia, the free encyclopedia)

What sounds best, fresh Nichi's or old BG's ? (don't answer!)
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Old 15th September 2010, 07:55 PM   #15
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Default When to toss?

General question to those of you who build alot of stuff: When do you decide to throw away formerly expensive passive components? I have a number of formerly good caps from old, gently used projects. Re-use or throw away? (e.g. 8-12 year-old Auricaps, KimberKaps, MIT multicaps, etc.) They seem so totally sealed that it's hard to imagine much chemical or physical degradation could occur within the dielectric! ...but I know so little when it comes to hard core 'lectronix!
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Old 15th September 2010, 08:13 PM   #16
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Originally Posted by francolargo View Post
General question to those of you who build alot of stuff: When do you decide to throw away formerly expensive passive components? I have a number of formerly good caps from old, gently used projects. Re-use or throw away? (e.g. 8-12 year-old Auricaps, KimberKaps, MIT multicaps, etc.) They seem so totally sealed that it's hard to imagine much chemical or physical degradation could occur within the dielectric! ...but I know so little when it comes to hard core 'lectronix!
Film caps are generally fine over time. It is possible for them to be damaged with usage, however.

http://www.illinoiscapacitor.com/pdf...itors_film.pdf

Electrolytic caps dry out over time, which is bad. They can often be reconditioned for use, however.

Electolytic Capacitor Application Guide :: DuraCap International Inc. .::. 519-539-4891 (end of document)
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Old 16th September 2010, 04:41 PM   #17
needsp is online now needsp  United Kingdom
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HI Russ

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qusp,

HFE is not that critical in the application because of the CFP. Still it won't hurt anything at all to use higher HFE. Give it whirl. just keep in mind, there is a filter cap there that will swamp most of the effect of the higher HFE. It is much better to have lower noise than high HFE in this application.

Russ
I don't understand "a filter cap there that will swamp most of the effect of the higher HFE". Do you mean the 1n rather than the 15n cap, and why is this? I thought the 1n cap was just to lower gain at at high frequencies and so increase stability, and would have little effect on hfe in the audio band. What am I missing . And wouldn't a higher hfe device further lower the input impedance of the CFP?

Be gentle- I mainly work with valves!

Thanks

Paul
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Old 17th September 2010, 02:16 AM   #18
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The 1nf cap is a form of compensation. It basically reduces the gain at high frequencies as you said. The main point is that the transconductance is already so high at audio frequencies that increasing the beta on the BJT is going to have little effect. Still there is absolutely no harm in using high beta BJTs, just be sure they remain low noise types. That is the most important attribute here.

I hope that clarifies things a bit.
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Old 20th September 2010, 10:08 PM   #19
needsp is online now needsp  United Kingdom
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Hi

OK, I've tried out a few ideas, and have come up with a couple of modifications to the already excellent Legato, which, to my ears, are very worthwhile.

Firstly replace the BC550B's with 2SC2547E. This has very low noise- about as good as it gets- and about double the hfe. It's also reputedly very linear when used in common base mode. Whatever the reason, it does the business. With BC550B fitted piano from CD was the best I've heard (piano on CD has always sounded wrong to me). With the 2SC2547E fitted, it's better still!

I matched the transistors for hfe, though the batch I have were probably close enough for this to be unnecessary. Note the pinning of these devices is bce, not cbe, so some lead bending and insulation is needed. The biggest problem is that-like all good things it seems- this transistor is no longer made, though it's not too hard to find. 2SC2240 is a near equivalent, though whether it sounds as good I don't know. BTW I also tried replacing the BS250 Mosfets with the 2SC2547E's complimentary pair, the 2SA970. This worked, but- as I've found in similar applications in the past- the combination is unstable, and generates program-related frying noises and clicking. Avoid!

Perhaps even more effective is fitting the current mirrors with constant current sources. To achieve this I replaced R19 and R22 with a CCS composed of cascaded BF245C and BF245A JFETS (see attached). A value of 68R for the resistor gave the required current- 2.7mA, and little adjustment was subsequently required to reset the input voltage to 1.70V (one half AVCC). Leave the circuit to stabilise for a few minutes before final adjustment.

Well, there you go. Apologies if you don’t hear what I hear!

Paul
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Old 21st September 2010, 04:15 AM   #20
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Quote:
Originally Posted by needsp View Post
Perhaps even more effective is fitting the current mirrors with constant current sources. l
If this were a single ended circuit you would be totally correct that adding CCS would help quite a bit. In fact with an SE DAC it will help significantly.

But in this case, with a truly balanced source, both the upper and the lower half of the circuit are already a constant current source/sink by virtue of the symmetrical nature of the circuit. In other words, the circuit is always in relative stasis. One side of the circuit is always counterbalanced by the other, so that the entire circuit is always in balance. Any impedance effect at one half is countered by an equal and opposite effect at the other. I had actually tried simply adding a cascode(in the form of a ccs), but I actually found this to reduce dynamic range far too much. Essentially what you have done is add a complex cascode.

I think you have probably been reading I/V threads that concentrate on single ended DACs.

As for the transistor choice, it is certainly a good part, and worth using, but not likely to yield astonishing results. I only say this because the noise is already vanishingly low. Still there are dozens of great transistors that will work very well. That certainly would be one. I would choose one with the same pin-out as the stock part.

Good work with your experiments. I hope my constructive criticism does not dampen your enthusiasm.

Cheers!
Russ
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