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Old 7th February 2005, 12:53 AM   #21
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Default Re: Bypassable

Quote:
Originally posted by Panelhead
I use a transformer coupled preamp, can I bypass these caps?
George
Probably not. Not only does the input cap establish a highpass with the tripath's inverting opamp input impedance, it blocks the dc, approximately +2.5v on the input, generated on chip, needed for its single rail operation.
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Old 7th February 2005, 02:15 AM   #22
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Default SI amp, high impedance phones, and ultrasonic noise

panomaniac,

A number of people are (or are considering) using the SI amp to drive headphones. With a 120 ohm or 300 ohm load, the SI's filter becomes pretty ineffective. There is a rise in it's response, reaching a peak of +10db at about 53kHz. I've been cautioning people about the ultrasonics and possible hearing damage that may result from listening to phones with this amp -- until we collectively answer a bunch of questions. For example, the Sennheiser HD-650 phones (300 ohm) are pretty much flat up to 35kHz, so you'd be listening to 35kHz noise that's 6db higher than the music level.

Check out these threads to see how the various arguments have developed:

http://www6.head-fi.org/forums/showthread.php?t=87285
http://www6.head-fi.org/forums/showthread.php?t=103035

If you can investigate this further, or shed some light on these issues, please let us know your results.

- Eric
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Old 7th February 2005, 02:41 AM   #23
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Default Re: SI amp, high impedance phones, and ultrasonic noise

Quote:
Originally posted by Eric Weitzman
A number of people are (or are considering) using the SI amp to drive headphones. With a 120 ohm or 300 ohm load,.....
Prior to dealing with issues of the load dependancy of the 2nd order lowpass output filter [see the tripath 2024 pdf for an explanation and equations] and its effect on the filter's 'Q', confirm that both the headphones and cabling to be used do not have a common ground. The amp is a btl (bridge tied load) with a single rail, and none of its ouputs sits at ground; it will not work if the phones or cable are wired 'common ground'.
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Old 7th February 2005, 02:47 AM   #24
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Default Bass responce at differeing loads

I did check the bass roll off at some higher levels. At near max output (8V P-P), there is a little more bass drop, it is down -10.5 dB versus -9 dB at levels around 1 watt.

There seems to be only the very slightest difference in level between 4, 8 or 16 ohms. You'll have more residual ultrasound at the higher impedance.

I agree with Motherone that it is probably the input caps and resistors that are making a hi-pass filter. I will try to bypass the stock units with outboard components of the correct values.

Maybe the input pot has something to do with it? Acting as part of the LC hi-pass, perhaps?

Aloha,
MM
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Old 7th February 2005, 03:54 AM   #25
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I'm almost certain it's a function of the input capacitor, or something related on the input of the amp. There is an input inductor (L1 on top and L2 on the bottom), as well as C3 (bottom) and C4 (top) before it hits the 20k input resistors, R2 (top) and R1 (bottom).

If you look at page 7 of the TA2024 PDF, you'll see that it's on .2 to .4 db down, depending on the impedance it's driving.

I suspect that the input capacitors are the culprit, though. They're so small that I have a hard time believing they're anywhere close to what's on the example on the datasheet.

I would suggest jumpering C3 and C4, and adding your own outboard input capacitors and see if that makes a difference.

Also, it's probably possible to add an external lowpass filter for the amp if ultrasonics are a concern.
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Old 7th February 2005, 05:30 AM   #26
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Default Re: SI amp, high impedance phones, and ultrasonic noise

Quote:
Originally posted by pmkap
Prior to dealing with issues of the load dependancy of the 2nd order lowpass output filter [see the tripath 2024 pdf for an explanation and equations] and its effect on the filter's 'Q', confirm that both the headphones and cabling to be used do not have a common ground. The amp is a btl (bridge tied load) with a single rail, and none of its ouputs sits at ground; it will not work if the phones or cable are wired 'common ground'.
This is (mostly) reasonable advice. However,

1. If you review the referenced threads, you will note that the need to recable the headphones to separate the common ground was made known to people who were unaware of this.

2. You will also note that the spice model and results that I posted contain the reference circuit from the data sheet. Also, the data sheet is referenced (with it's URL) in several posts.

3. Rewiring headphones is not a prerequisite for modeling the circuit, understanding the effects of ultrasonics below 100kHz on human hearing, or making measurements using dummy 120 ohm loads that don't share a common ground.

4. The amp does indeed work if the two channels are tied together. What it sounds like is another matter entirely, known only to those who have tried it
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Old 7th February 2005, 10:56 AM   #27
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Default Re: Re: SI amp, high impedance phones, and ultrasonic noise

Quote:
Originally posted by Eric Weitzman
4. The amp does indeed work if the two channels are tied together. What it sounds like is another matter entirely, known only to those who have tried it
Well I have tried it, by accident. With the grounds tied together into an 8 ohm load, the amp is very unhappy. Mostly it clicks strongly, a sort of cry for help. Is that what you mean by "the two channels tied together." ?

I do not know what happens with a common ground on a high impendance load, but I can't see why it would be different.

Aloha
MM
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Old 7th February 2005, 06:27 PM   #28
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Default Re: Bass Shy

Quote:
Originally posted by panomaniac
We all know that the cute little Sonic Impact is bass shy.

Measurements show that it is. Have a look at this graph.
Level was 2V P-P for this measurement, but seemed the same for all levels.

This chart is not yet on my site. You saw it here first!

Aloha
MM
That's funny! I built a couple of 4-ohm fullrange speakers for my SI, and solicited help on this forum to design the required highpass filter to prevent overexcursion on these speakers:
Need help with PLLXO High Pass filter for full range's!

Turns out the required filter is almost exactly what's shown by your testing (a 50Hz HPF around .5 Q).

The current demands into a 4-ohm load are suprising as well, and suddenly make my 1700ma supply look a little anemic.

Thanks for the testing, though!
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Old 7th February 2005, 08:44 PM   #29
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Default Re: Bass Shy

Quote:
Originally posted by bwbass
The current demands into a 4-ohm load are suprising as well, and suddenly make my 1700ma supply look a little anemic.
You may be alright with 1.7 amps. It depends on what you listen to.

The bass response chart is now on my site, along with a page called "Musical Power." Check it out to see how much power you might actually be using. As long as your PSU has a big cap on the output for peaks, you should be OK. Please have a look and let me know if you have any questions.


http://www.michael.mardis.com/sonic/start.html

Aloha
MM
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Old 7th February 2005, 08:52 PM   #30
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Default Musical Power

I have added a page to my Sonic-Tripath site called

Musical Power
http://www.michael.mardis.com/sonic/start.html

On this page I explain how little musical power you can really get out of the Tripath chip, if you want to avoid clipping. Some nice waveforms are included showing average levels in various types of music.

Please have a look and let me know if you have any questions, ideas or disagreements.

Aloha
MM
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