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Old 26th April 2001, 08:41 AM   #1
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Hello all

I am new to the forum and (at the risk of boring everybody with another thread about Nelson Passes Zen amp) would like to ask a few questions:

1) Who has actually built one of these things? (I would like to hear your listening impressions, gripes, praises etc.)

2) Are mosfet substitutions easy to make/does the circuit need much modification (if any)?. Has anybody tried substitutions and what were they?

3) Does anybody know anything about BUZ350 power mosfets? I got on to a few of them really cheap surplus and was wondering if they would be suitable for the zen amp. I think they're made by seimans???

thanks

Dan

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Old 27th April 2001, 01:44 PM   #2
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Sound: Good imaging, smooth, not harsh at all. .5db down at 20khz, but can be raised w/ feedback if needed. Or, you can raise the value of the input and output caps, then bypass them with high quality film types. Female voice tests show accuracy in the midrange. Very liquid. One second there's no sound and then WHAM! Very quite when used w/ coils mod from second article.
It can be weak in the lower end, but I think that with some music, the low end is there, but lacks control. Electric bass lines that are quick get jumbled up, if you know what to listen for.

It lends itself well to modification and part substitution.
Find the lowest possible voltage rated FETs to suite your needs and use higher current to drive them, like stated in the second article. I used IRF044 from Digi-Key. They are low priced TO-247 and are about 50 volts and plenty of amps. Perfect for the ZEN as it is designed.

If you're using the circuit board, mind your parts placement. I had problems w/ this at first. The silk screen is confusing; I'm not the only person that had a problem w/ it.

I don't know anything about BUZ's. I only use IRF types.

You can see my ugly looking, but beautiful sounding ZEN amp on my site. Click on the little red house below!

Any questions, I'll be happy to help.

Good luck!

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Old 1st May 2001, 03:19 AM   #3
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Default Thanks Vince

Well I'm glad somebody replied. The description you gave of the sound quality has inspired me to get this project underway ASAP. I'm taking your advice and not using the BUZ350's (Vds = 200V) and am looking for lower voltage higher current mosfets. I have a couple more questions you or perhaps others can answer.

1) I read in a previous post the frequency response of the zen only goes up to about 15KHz, why is this so?

2) How does a thyristor soft start circuit work?/what does it look like?/do I need one for a 2 x 10W zen amp?

3) I have looked at the Lidstrom improved Zeni circuit because I want to use all N type mosfets. Does anyone know if it is missing a ground on the constant current source side or am I just reading it wrong?

Thanks again

Dan
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Old 1st May 2001, 01:08 PM   #4
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The frequency actually is down .5db at 20khz. Most adults can't hear at that freq. A child might, but even then... But, what happens is if there are any anomalities above 20khz, they could make their way into the audible band. I don't hear any problem with the ZEN. It's not very bright in the upper end. I have titanium dome tweeters in my speakers. Which are bright sometimes or metallic sounding. It is no different using the ZEN w/ my speakers. There is plenty going on in the upper octives, it's just not a harsh amp, which I think is good. Some people use it as a mid and tweeter amp.
(Remeber, most CD Players only play up to 20khz, the rest is filtered out.)

I see what you're saying about the Lidstrom mod. I don't know the answer to that. I would follow the schematic. I don't think the mod makes a difference in the sound quality, but I could be wrong. It does give you the option of using only N-CH MOSFETS. If that's what you want, then you're in there!

The thyristor circuit is simple. Look at the http://www.passlabs.com site under downloads > service manuals. Find the Alephs and look at the power supplies. Pass uses 2 in parallel after the plug. When the TH start to warm up from the current passing through them, they reduce in resistance. It restricts the on-rush of current. It won't hurt to usi it on the ZEN. I don't have one. I can see my woofers move forward when I turn the ZEN on.

This is a project worth trying.
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Old 1st May 2001, 09:57 PM   #5
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The more I think about the more I get in to this project. I have almost all the parts except the thyristors, mosfets and a PCB. At the risk of blowing up the house and everything in it I have decided to design my own, this could take a while. What I might do is post the design and see if anyone on the forum can see anything wrong with it before I proceed with the construction.
Vince you said you had metal dome tweeters, could you describe the rest of your speaker system so I can compare it with mine (I want to know if its zen friendly).
At present I have a set of ported mini monitors which I designed and made myself. They use a vifa PJ17 woofer and a 25mm vifa metal dome tweeter (I can't remember the code). At this stage they are highly experimental and therefore look pretty bad but sound really good (I think so anyway). The crossover is of my own design and based on a 2nd order LR. I have been experimenting with different crossover components and am amazed at the effect they have on the sound. Also I made the DIY cat5 speaker cables from chris V's site and they perform very well.

Dan
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Old 2nd May 2001, 12:19 AM   #6
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Quote:
Originally posted by ding
2) How does a thyristor soft start circuit work?/what does it look like?/do I need one for a 2 x 10W zen amp?

[/B]
I think you mean thermistor instead of thyristor here :-)
(I have heard of triac based soft-start circuits from Bryston, for example).Check this link for some thermistor info and possibly samples: http://www.ametherm.com/Inrush_Current/welcome.html

The short story is the device has "high" resistance when cold. This provides current limiting which can provide soft-starting for a power supply. The device warms up as current flows through it. After a brief time, the device resistance drops to a low enough value as to no longer limit the current.

Michael
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Old 2nd May 2001, 01:48 AM   #7
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Thumbs up Thanks Michael

Do you have any Idea how to go about sizing thermistors for inrush current limiting applications?

Dan
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Old 2nd May 2001, 04:32 AM   #8
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Default Mosfet mix n match

I have procured some IRFP150's (Id = 42A, Vdss = 100V, PD = 160W). These are quite expensive (for me anyway) so I only bought 2 for now and was hoping to use these for the gain stage and the BUZ350's (Id = 22A, Vdss = 200V, PD = 125W)for the CCS. Is it possible to use different mosfets for these different applications. If not I will stop being tight and go and get 2 more of the IRFP150's and build the Lidstrom ZENi (with some changes).

Dan
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Old 2nd May 2001, 12:44 PM   #9
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You can mix the different FETS. From all my reading, the only thing to be mindful of is the voltage rating, which should be low as possible, but not too close to the rail voltage, the current rating (your 150's look good), wattage dissipation of the signal FETs.

The CCS FETs should meet comfortable ratings for your application: wattage dissipation, current, voltage.

Your speakers sound a bit like mine, but you used better parts and a 12db slope. I have an OEM 6-1/2" polyprop. woofer and 1" titanuim dome. I believe I have an OEM JBL tweeter. They sound off at about 90db spl/1m. I built them in college for about $70 for the pair. Mine aren't finished either. It time to design something new. I'm trying to enter the DIY 2001 speaker contest. This should motivate me to design something killer!

If you use the circuit board from Old Colony Sound Labs, let me know. I think it's tricky to use. All the transistors go in the board in reverse. Not like shown in the silk screen. I had the most problems w/ this. Like I said let me know. I'll walk you through it if you have questions.

good luck!

Vince
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Old 3rd May 2001, 09:53 AM   #10
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Default New Thread

Vince,

I have posted a new thread about my PCB design for this project. I would have added it to this thread but I fear you are one of the few people reading my posts. I would appreciate it if you would have a look at my design as you have been helping me more than anyone on the forum. Let me know if you want me to email it to you and if so in what format.

Dan
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