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Old 10th July 2007, 08:10 AM   #11
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Default Re: Re: Re: Passive pre.... what's going on with this one?

Quote:
Originally posted by AndrewT
Hi George, did you lose a decimal place or could you explain this result?

OOOOOps -3db @ 2hz
way to quick on the buttons, calculator could'nt keep up!!!!!!!!!!!!!! or failing eyesight, I go with the calculator not being fast enough, makes me feel better.


Cheers George
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Old 10th July 2007, 08:27 AM   #12
AndrewT is offline AndrewT  Scotland
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Hi George,
is 1.6Hz good enough for a pre-amp?
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Old 10th July 2007, 08:57 AM   #13
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Quote:
Originally posted by AndrewT
Hi George,
is 1.6Hz good enough for a pre-amp?
I prefer to DC couple, so long as there are no nasties (dc offsets)present, and if there are, I'd rather work hard to get rid of the dc offset than use a cap.
But if you must have a coupling cap, then anything below -3db @ 3hz is fine.
But like I said I prefer dc coupled, as no matter how good the cap, no cap is better, a slight softening of leading edge to transients if it's a good cap, and just down right stodgy and veiled if it's a bad cap.

Cheers George
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Old 10th July 2007, 02:04 PM   #14
rabbitz is offline rabbitz  Australia
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Default Re: Re: Re: Passive pre.... what's going on with this one?

Quote:
Originally posted by AndrewT
Rabbitz,
we do need the cable lengths and power amp input capacitances to complete this analysis and find your optimum settings.
Hi AndrewT
The path is like this:
CD 47uF, 100R
Cable 1m @ 32pF
Pot A20K
Cable 0.3m @ <10pF
Power amp 470nF, Zin 42K (I know the cap sounds low, but Hugh knows how to design these things)

Hi George
All the resistors and cap have been removed and yes, it does sound better. The clarity and soundstage has improved plus the dynamics have been retained which is a surprise for me as all my other passives tended to lose dynamics. The bass has lost a tad of punch and drive but the extension is there so it's not rolling off. These speakers have only had it's woofer changed from active + power amp to passive xo so there is still some tweaking to go and I can get the drive back. Why remove active you say? I want the amp to do all the range as it is that good and am willing to lose some punch but gain detail, quality and finesse.

So the passive has beat all the active pre's thus far except in a slight loss in bass drive but that's easy to fix by tuning the speaker to suit. The gain in all other areas make it worthwhile and this has been the first setup I've had that has resonded favourably to a passive bar a LM3875. I have yet to try a buffer (Zout <30R) between the CD and the pot and that will happen later in the week.

This process will go on for quite a while as fine tweaking and matching takes it's time when you change any component in the system.

I really appreciate your comments and ideas. Passive pre's, so simple yet so hard.
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Old 10th July 2007, 02:36 PM   #15
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This guy has some good info on cables- tweek info

Even though he doesn't like steel wire, I've been very happy with good old Belden 8421 coax. It's low capacitance and the foamed dielectric isn't too awful. I do believe dielectric is the critical item, not conductor material, so recommend going to a Teflon dielectric if you can afford it.
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Old 10th July 2007, 03:43 PM   #16
AndrewT is offline AndrewT  Scotland
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Default Re: Re: Re: Re: Passive pre.... what's going on with this one?

Quote:
Originally posted by rabbitz
CD 47uF, 100R
Cable 1m @ 32pF
Pot A20K
Cable 0.3m @ <10pF
Power amp 470nF, Zin 42K
no RF filter in the power amp?

Gootee,
can you run the numbers on this after we get confirmation of the RF filter?
and then substitute a 5kpot for the 20kpot.

I can see that the 470nF & 42k are cutting off the bass early. I wonder if the amp can respond to a change here?
I would like to see this filter dropped by two to three octaves, but the bottleneck might be inside.

Rabbitz,
the buffer might do more good after the pot rather than before, but that also depends on whether you will ever change those cable lengths to make volume adjustment more convenient or re-locate the power amp beside the speakers.
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Old 10th July 2007, 04:28 PM   #17
rabbitz is offline rabbitz  Australia
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Conrad
This is the cable I'm using http://www.oyaide.com/e_audio/audio_...iles/pa-02.htm and I think the cable itself is made by Furukawa who are no slouch is this field. It's very similar in concept to Vampire CCC-II which is also a cable I'm fond of.

AndrewT
The amp certainly does not have a problem in the bass area with -1dB points at 15Hz and 85KHz and the bass is one of the strong points in the reviews and I have to agree. It's just a matter of seeing if it's a good candidate for a passive pre, and from what I've heard in the last 24 hours, I'd have to say yes. A 10K pot would be better I think but the 20K is doing well.

The circuit information however is proprietary and belongs to Hugh Dean so there is no information on the RF filter and I respect that. This is the amp I'm referring to. http://www.aksaonline.com/products_2_3.html

I'd say the cable lengths would remain constant due to the configuration of the room and rack and if the power amps were ever moved near the speakers, then it would have to be active pre for sure. You're right and read my mind as I was going to try the buffer before and after the pre.

A slight bit of tuning of the speaker port and maybe some crossover tweaks will return the bass drive to where it was with an active pre (we're not talking much here as it's only just noticeable) which is not half bad for a passive pre..... I'm impressed. This power amp has responded extremely well and has given better results than the heaps of others I've tried with a passive. It's easier to tweak a speaker to suit an amp than the other way around.

Thanks for all your advice.
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Old 10th July 2007, 09:15 PM   #18
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Quote:
Originally posted by rabbitz

The amp certainly does not have a problem in the bass area with -1dB points at 15Hz and 85KHz and the bass is one of the strong points in the reviews and I have to agree. It's just a matter of seeing if it's a good candidate for a passive pre, and from what I've heard in the last 24 hours, I'd have to say yes. A 10K pot would be better I think but the 20K is doing well.

The circuit information however is proprietary and belongs to Hugh Dean so there is no information on the RF filter and I respect that. This is the amp I'm referring to. http://www.aksaonline.com/products_2_3.html

Thanks for all your advice.
Rabbitz, your right in assuming by dropping the passive pot from 20k to 10k you may see an improvement in bass attack, you are well known in the Newcastle area, I have sent a few of my Lightspeed Attenuators up there maybe you could ask around to borrow one, they are the equivalent of 7k-10k constant input and output and will sound even better than any passive pot.
Lightspeed Attenuator a new passive preamp

Cheers George
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Old 11th July 2007, 02:27 PM   #19
rabbitz is offline rabbitz  Australia
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George
I've been following the Lightspeed thread since the start and very intrigued.... been thinking about getting one for quite a while. Been working out how it goes in my existing pre case, remote volume etc.

I still have quite a bit of testing to do with a few different variations. I can always drop the value of the pot by resistors from the input to the wiper but easier to try another pre I have with a 10K Alps black beauty.... I'll disable the active circuit.

Most of the bass variation has been sorted and was caused by the removal of the plate amps when the new amp took over. Extra volume in the speakers which in turn pushed the Fb down so was just a matter of reducing the port length.... easy.

Well known in Newcastle... hope it's not the law enforcement agencies.
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Old 11th July 2007, 09:10 PM   #20
gootee is offline gootee  United States
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Default Re: Re: Re: Re: Re: Passive pre.... what's going on with this one?

Quote:
Originally posted by AndrewT

CD 47uF, 100R
Cable 1m @ 32pF
Pot A20K
Cable 0.3m @ <10pF
Power amp 470nF, Zin 42K
no RF filter in the power amp?

Gootee,
can you run the numbers on this after we get confirmation of the RF filter?
and then substitute a 5kpot for the 20kpot.
Hi Andrew,

No problem, except that I'm not sure about what the configuration is, exactly.

Would it be: Source, then a series 47uF, series 100R, 32pF to gnd, 20k (or 5K) pot, wiper out, to 10pF to gnd, series 470nF, 42K to gnd ?

And if there was an RF filter, I assume that it would go either before or after the 470nF (and might consist of a few hundred series ohms and a few hundred pF to gnd, and then maybe another few hundred series ohms).

Correct?

OT: I saw the "The Last King of Scotland", recently. Interesting movie, in some ways. It's about Idi Amin's young Scottish doctor. (Apparently, Amin loved "all things Scottish". I never would have guessed THAT.)

- Tom Gootee

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