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Old 5th August 2008, 13:51   #31
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Originally Posted by DerAlte View Post
Why should the transistorized intermediate amp deteriorate SQ? I think you are being empirical - there are no data points here.
Though I did not measure anything but I definitely say details get lost from preout and we two (me and my brother both) realized this instantly.

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Not a good idea to remove components from the signal path without examining their role.


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Noise is affecting your signal since early in the signal path the SNR is bad - not because noise is more, but because signal is less. What was OK with the Toshiba chip (there is an op-amp in the input stage, whose gain you have ignored) is not OK without it.
This is exact reason SNR at that point is bad. Because good SQ but with free gift of noise (which some of my colligues who listened to it but could not recognize).

So 1 ) what if I improve signal strength by another pre-amp
or 2 ) same time add those inline resistors too?
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Old 5th August 2008, 13:59   #32
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omisra, do you remember the name of the DAC chip ?
It was "LV3311", I am not able to find anything for that. I will look closely at high res pics in my personal desktop and tell more about it. There was no manufacterer name too. It looks some low cost fabrication.

Last edited by omishra : 5th August 2008 at 14:02.
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Old 5th August 2008, 14:05   #33
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Is there any space within the HU to place the pre amp? else there isnt much of a point, placing the pre amp outside the HU. Maybe you should have a look at the section immediately preceding the RCA outputs and see if that can be tweaked to do the job
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Old 5th August 2008, 14:23   #34
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Originally Posted by omishra View Post
So 1 ) what if I improve signal strength by another pre-amp
or 2 ) same time add those inline resistors too?
Adding resistor may not help unless you use some sort of noise filter.
It would be good to add some low noise op-amp. (sorry for interrupting between the conversation - but i am having fun in this thread, its a great DIY job)
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Old 5th August 2008, 14:31   #35
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Omishra, this is the best DIY thread on Team-BHP. Bravo!
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Old 5th August 2008, 14:57   #36
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Thanks Sam! Its true that learning never ends.
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Old 5th August 2008, 15:10   #37
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Originally Posted by mohang_j View Post
Adding resistor may not help unless you use some sort of noise filter.
It would be good to add some low noise op-amp. (sorry for interrupting between the conversation - but i am having fun in this thread, its a great DIY job)
Noise level is constant and not related to audio signal level. Means inline resistor suppress' everything at that level including audio. Now increase audio signal using volume control and get it above noise level. OEM TOSHIBA chip was also facing noise issue and in this way it was suppressed. So you have to increase audio level at > 50% to get decent sound level.

Now thinking over TL072 based buffer for this and also using the stock 100K resistor inline.

OK getting near to it.
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Old 5th August 2008, 15:41   #38
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Originally Posted by omishra View Post
Noise level is constant and not related to audio signal level. Now thinking over TL072 based buffer for this and also using the stock 100K resistor inline.
That is if you add 100k res will reduce signal and noise but when you amplify it again noise will increase with signal so use op-amp with some noise filtering circuit.
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Old 5th August 2008, 17:00   #39
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Though I did not measure anything but I definitely say details get lost from preout and we two (me and my brother both) realized this instantly.
If anything is missing, it will be a whole band, not 'details'. IMHO the best way to check would be feeding a square wave and looking at the output on a scope. If the pre-out is lousy as you make it out to be, you will see the corners rounded and the verticals slopey. A transistor circuit is generally unlikely to misbehave at HF - you are forgetting a 40 year history in SQ using transistors. The 'sharpness' of a square wave is richness of harmonics of HF sinusoids (go ahead - do a spectral analysis). Rounded corners and slope of the edges is indicative of loss of harmonics.
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So 1 ) what if I improve signal strength by another pre-amp
or 2 ) same time add those inline resistors too?
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Originally Posted by mohang_j View Post
Adding resistor may not help unless you use some sort of noise filter
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Originally Posted by omishra View Post
Noise level is constant and not related to audio signal level. Means inline resistor suppress' everything at that level including audio.
Mohan: cardinal rule in hi-fi audio - don't add elements that take away something from the signal. You add a noise filter - it will take away some amount of highs.
Mishra-ji, the in-line resistor effectively makes the source a lower impedance than the drain, minimizing the current drawn by the drain. Lower current = lower noise! No, the current dependent part of the noise increases with signal level.
For the moment, forget about noise, just use an intermediate amp to jack up gain. Good amps use at least 5 stages of amplification before the final drive, all the while ensuring the transistors (even the ICs have transistors, yar) are operating in the linear region of the gain curve. As soon as you start loading it to the non-linear region, you get more noise and distortion.
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Old 5th August 2008, 20:09   #40
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If anything is missing, it will be a whole band, not 'details'.
I meant to say only vocal range (Mid) is played loudly and HF LF are dampened.

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Mishra-ji, the in-line resistor effectively makes the source a lower impedance than the drain, minimizing the current drawn by the drain. Lower current = lower noise! No, the current dependent part of the noise increases with signal level.
For the moment, forget about noise, just use an intermediate amp to jack up gain. Good amps use at least 5 stages of amplification before the final drive, all the while ensuring the transistors (even the ICs have transistors, yar) are operating in the linear region of the gain curve. As soon as you start loading it to the non-linear region, you get more noise and distortion.
DerAlte, I agree all of these points. Hence I did not add .01uF from signal to ground. I could have got rid of HF noise. But I restrained myself. I know that transistors are not only culprits.

Anyway this noise is minor issue and I am only able to sense it because I wanted it to be perfect. This issue anyway does neither hamper the improvements I achieved nor it takes away listening pleasure. I will study it further and resolve carefully.

You know the strange behaviour of this noise - "If I make equilizer setting BASS=0 and TREBLE=0, is minimizes beyond recognition. On other side of either setting +/- it shows up." In this max level only it can be described as - at level 3 out of 45 of audio level audio and noise are equalled.

Everybody, thanks for clues and inputs. Please keep writing your thoughts.
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Old 12th August 2008, 21:52   #41
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Futher improvements -

I designed new buffer to be added in between DAC output and my switching AMP.
This is TL072 dual OP AMP with 3MHz Bandwidth. Each channel is now designed for Gain = 2 ( i. e. 6dB ). This OP AMP needs dual power supply. So added artificial ground at Vcc/2 ( apporx 6 volts). So this will operate at +6V (actual 12V), 0 (actual 6V) , -6 (actual 0V).

Name:  pre-Amp-sch-50.bmp
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Again another home brewed PCB

SX4 OEM HU: By pass surgery for Amp section-dsc01021.jpg

SX4 OEM HU: By pass surgery for Amp section-dsc01023.jpg

SX4 OEM HU: By pass surgery for Amp section-dsc01024.jpg

Tested successfully on the desk, now time to fit this inside HU. This weekend could not do it due to relatives visited us.

Noise issue is tracked down and will be fixed with this. This was due to interference of DAC's internal clock with my AMP's switching frequency (which settles at ~560KHz-1300KHz in AMP with some harmonics). This was polluting power supply of DAC and all other HU controls. So I need to add inductor & some filter between DC supply lines of AMP and HU. I bought ferrite rings of T18 type to build filter toroidal coils of 15A rating. Additionally all HU control chip's power supply will get 0.1uF noise filter cap. All of this due to presence of switching frequency. Normal class AB amp will not give this troubles. But its ok for for me due to switching AMP's efficiency and good sound quality.

And that so called 100K inline resistor in audio path was required to provide noise immunity between DAC & AMP. But anyway now buffer will provide some gain and that immunity too.

Waiting ahead!

Last edited by omishra : 12th August 2008 at 21:55.
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Old 12th August 2008, 23:12   #42
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Congrats for the efforts and the success,this is an astounding DIY.

The tech specs went over my head as well,but in lay man's language,Did you fiddle with the internal amp of the system?

Last edited by noidaboy : 12th August 2008 at 23:13.
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Old 12th August 2008, 23:45   #43
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Quote:
Originally Posted by greenhorn View Post
Anything other than the LM 324
I'm a big fan of OP275s. Try them instead.

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This is TL072 dual OP AMP with 3MHz Bandwidth.

Normal class AB amp will not give this troubles. But its ok for for me due to switching AMP's efficiency and good sound quality.
why go PWM at all. How much power (in db) are you gaining?

If you choose your speakers carefully you might be able to get the same SPL with a class AB amp.
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Old 13th August 2008, 08:11   #44
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Awesome. I was thinking so much to do DIY just for the 2 rear speakers and here you are doing complete upgrade on the OEM HU

And you designed everything yourself. Thats something great. Congrats.
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Old 13th August 2008, 09:18   #45
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+1 to navin's suggestion
Quote:
AM mode significantly reduces EMI generation since the output amplifiers are now operated in linear mode. Operating in Class B mode also reduces the TAA4100A efficiency especially at low to medium output powers. Due to this increased power dissipation, it is recommended that the AM mode is used for applications such as AM radio playback where the average output level is minimal and a switching amplifier would most effect radio reception.
From the datasheet. I'd recommend AM mode since you're running off 14.4V anyway (PWM mode makes a difference only for higher voltages), and the THD+N figures are lower for AM mode as well

Last edited by greenhorn : 13th August 2008 at 09:22.
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