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Old 7th December 2006, 02:11 PM   #4981
AndrewT is offline AndrewT  Scotland
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Hi,
Quote:
Originally posted by Chivvyp
Is ZIN the input impedance of the amp? I always had problems with working out whether this related to the power amp or the preamp or a combination of both.

With my amp modules I believe the impedance is around 22k but my volume pot is 50K.

I chose my attenuation to match 22k on the basis that I'd fit a 20K pot in the future.
I see the confusiom.
You have two Zin, the power amplifier Zin=22k and attenuator Zin~=50k.

But it's not that simple.
Firstly the attenuator sees the power amp Zin and this should roughly follow the ten times rule. For a power amp with Zin=22k the source impedance of the attenuator should be less than or equal to (<=) 2k2.
The maximum source impedance of an attenuator is about one quarter of the pot value. This results in a pot value of about 8k8. The nearest value is 10K giving a ratio of 22k/2k5~=8.8, close enough to ten times for our purpose.
The actual source now sees the pot as it's load. BUT, the load varies.
The pot is in parallel to the power amp Zin and as you change the volume setting the impedance of the parallel combination changes.

When the pot is set to minimum, Zin//0ohms and the series resistor is 10k so the load is 10k.
When the pot is at half volume (-6db) there is a series resistor of 5k and then 5k//22k =4k07 the source sees 5k+4k07=9k07. Not much different from 10k so we are alright. The actual attenuation is -7db but this is an adjustable attenuator and an extra db is of no consequence.
When the pot is set to maximum volume the series resistor is 0ohms and the lower leg is 10k//22k=6k88 and the load on the source is 0+6k88=6k88. Again not too much lower than the 10k maximum load. This works because we have stuck to the ten times rule (or close to it).

Your 50k pot and 22k power amp is outside the ten times rule.
Here the max load=50k and min load =15k3 a ratio of 3.2:1 cf previous 1.45:1

The 10k pot with 22k power amp has a minimum load of ~=7k. Using the ten times rule a suitable source impedance for driving this would be <=700r. Many modern sources fall into this category. The few that are 1k or 2k would be less suitable.

Does that clarify things for you?
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Old 7th December 2006, 03:25 PM   #4982
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Default Re: Ultra-Low noise 5V PSU

Quote:
Originally posted by pantera6



I can't recall where I got this circuit from, but it seems to be a good one for the 5V clock supply (and/or for DAC analog/digital supplies, Servo supply, Digital Filter analog/digital supplies).

Thoughts anyone?

I was reading LT Application Note 82 (that's one of the places you can find this schematic), and was interested in trying this regulator (and possibly making a negative version of it, too). Pantera (or anyone else, for that matter), did you ever get around to trying this one? I will probably breadboard it, and also try AD587, which is pin compatible.
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Old 7th December 2006, 03:57 PM   #4983
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The sch will be from the regulators pdf file.

Its ok - better than 78** thats for sure.

Discrete regs using op amps is the best way to get low noise, with the opamp driving a transistor or fet

Brent
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Old 7th December 2006, 05:04 PM   #4984
Chivvyp is offline Chivvyp  United Kingdom
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Quote:
Originally posted by AndrewT

Does that clarify things for you?

No!

It goes way over my head to be honest but it certainly gives me plenty to think about!

But thanks for the info. I need to consider it and go back to the specs for the amp. I think they reccommended a 20k pot but said 50K would be ok ( and I had one around).

Regards

Pete
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Old 7th December 2006, 05:17 PM   #4985
AndrewT is offline AndrewT  Scotland
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Hi Chiv,
if there is any specific para or sentence that loses you just shout.
We are listening.
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Old 7th December 2006, 06:01 PM   #4986
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If I were to upgrade the power on my DVD player, my goal would be separation more than noise performance. The PCM1738 has 3 analog +5V supplies and 2 analog grounds. Unfortunately, both are unified in the DV47ai, and that's pretty much what's diagrammed in the data sheet! As if such a lay-out can meet the >110db channel separation spec.!

I have some Black Gate 0.47uf NX-HiQ caps. Any point in using them to make at least separate PS bypasses?

I hate data sheets! The mfr gives circuit diagrams that are as simple and cheap as possible to use the part to sell more, but there's no way specs can be met with them. Nine times out of ten, products are then built exactly from the examples. Grrr, we get crap.
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Old 7th December 2006, 11:37 PM   #4987
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Great thread!

I am now really tempted to join you guys to mod a CD67se. Could somebody be kind enough to send me the schematic? It will be very much appreciated.

Regards,
Bill
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Old 8th December 2006, 12:57 AM   #4988
jksmurf is offline jksmurf  Hong Kong
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Quote:
Originally posted by HiFiNutNut
Great thread!

I am now really tempted to join you guys to mod a CD67se. Could somebody be kind enough to send me the schematic? It will be very much appreciated.

Regards,
Bill
Bill

See http://home.quicknet.nl/qn/prive/ra..../index_en.html for all the schematics you will need for your CD63/67SE. And if you blow it up can I have your 220V 67SE transformer :-)

k.
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Old 9th December 2006, 03:23 PM   #4989
Chivvyp is offline Chivvyp  United Kingdom
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Quote:
Originally posted by AndrewT
Hi Chiv,
if there is any specific para or sentence that loses you just shout.
We are listening.

Hi Andrew,

Just trying to reconcile all the different sources of information

I checked the specs for my amp modules, Zin (signal load input), min 14.8, typ 17, max 47.

So I guess I should have a 10K pot rather that the 50K I have currently.

Now, I need 18db attenuation. If I go to http://www.tnt-audio.com/clinica/diyattenuation.xls and put in a load of 10K that gives me values of 8k8 and 1k4. Is this ok or would I be better using something like 1k5, 200 as this would seem to lower the effective resistance to something nearer the original target of 17K/10 (or am I totally misunderstanding the way that this all works?

Regards

Pete
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Old 9th December 2006, 03:41 PM   #4990
AndrewT is offline AndrewT  Scotland
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Hi,
you seem to have understood it.
Select available resistor values. From the E24 range 9k1 & 1k3 will give about -18db.
I don't have access to E96 range values.

The effective minimum impedance seen by your source will be [9.1+1.3]//Zin.

I assume your source can drive that.

If you reduce the fixed attenuator values then you improve the power amp end by reducing the source resistance but on the other hand you reduce the load seen by the source.
How low dare you go?

Resistors are so cheap, it may be worth making up pairs of attenuated interconnects with effective impedances of 10k, 5k and 2k and listen to each.

When you have settled on an acceptable load then buying some expensive resistor types may be worthwhile. I tend not to recommend that route to Nirvanna, but others try exotica and report quality improvements.
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