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Old 30th April 2012, 07:06 PM   #1
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Default 8-Channel Gainclone Amplifier

I'm building this multi-amp to drive my DIY speakers. They are 4-way active dipole prototypes with MiniDSP crossovers.

Click the image to open in full size.

Currently they are powered by 8-channel ClassD amps. They're great except not liking the 2 ohms nominal tweeter impedance (paralleled, the back tweeters are not visible). I did a study on the distortion between gainclone and ClassD before at the acoustic output here.

The 8-channels gainclone amps are based on AudioSector 4780 kit:

Click the image to open in full size.

As seen above, I'm using no-fuss components sourced from local provider here (Jaycar). I did the effort, however, to painstakingly measure the resistors individually before using it.
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Old 30th April 2012, 08:15 PM   #2
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nice looking speakers
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Old 30th April 2012, 09:08 PM   #3
boris81 is offline boris81  United States
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Why not save yourself the extra work and just add 2 Chipamp channels to your existing Class D amp? You can match channel levels in the miniDSP.

Do you expect them to sound very different?
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Old 1st May 2012, 10:55 AM   #4
AndrewT is offline AndrewT  Scotland
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six speakers in one tower. Use 6 amplifiers. That will require two 6ch amps for stereo.
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Old 3rd May 2012, 01:51 PM   #5
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An amp for each driver. I like your thinking

I have spare place for 4 more channels. A future 12-channels amp!

Click the image to open in full size.

re: why not just use the GC for tweeter and the rest class D? Well I don't want complexity. Multi-PSU and all. But it would have been workable and no difference in the sound
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Old 3rd May 2012, 04:59 PM   #6
jb74 is offline jb74  United States
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Default How much transformer ?

Have you decided on the transformer to power your new creation ?

jb74
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Old 4th May 2012, 07:33 PM   #7
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Quote:
Originally Posted by gainphile View Post
Currently they are powered by 8-channel ClassD amps. They're great except not liking the 2 ohms nominal tweeter impedance (paralleled, the back tweeters are not visible). I did a study on the distortion between gainclone and ClassD before at the acoustic output here.
Class D amps are especially happy with low impedances. The trouble at the top end lies in the passive low pass they need to convert the PWM output back to a sine wave. The filter will only work perfectly at one specific impedance. The solutions are either to adapt the filter to the load the actual speakers pose or to linearise the speaker load to the impedance the filter was designed for.
Often the amp designers assume that the load is resistive for the filter design. The actual frequency-dependant impedance of a real loudspeaker will then warp the frequency response in an undesired manner. That is why many people report that class D amps sound harsh, edgy or even piercing. An impedance linearisation for the tweeter solves that issue.
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Old 5th May 2012, 04:41 PM   #8
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The amps are easy enough to wire, although not as easy as ClassD

What is critical is the star grounding. If done incorrectly the output will hummmmmmmm...

Click the image to open in full size.


Quote:
Originally Posted by pacificblue View Post
Class D amps are especially happy with low impedances. The trouble at the top end lies in the passive low pass they need to convert the PWM output back to a sine wave. The filter will only work perfectly at one specific impedance. The solutions are either to adapt the filter to the load the actual speakers pose or to linearise the speaker load to the impedance the filter was designed for.
Often the amp designers assume that the load is resistive for the filter design. The actual frequency-dependant impedance of a real loudspeaker will then warp the frequency response in an undesired manner. That is why many people report that class D amps sound harsh, edgy or even piercing. An impedance linearisation for the tweeter solves that issue.
You are correct, but I don't think Sure TA2050 which are harsh. In fact my study above proved the distortion is the same as gainclone. Yes the frequency droop can be fixed by changing the output filter. But in my case it's much simpler to correct it at the DSP.
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Last edited by gainphile; 5th May 2012 at 04:50 PM.
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Old 5th May 2012, 07:02 PM   #9
boris81 is offline boris81  United States
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It looks like a great project! I would love to build a multichannel amp one day for my active 3-way speaker. My skills are not up to point where I'm comfortable to tinker around with things that run on mains voltage. For the moment I decided to buy a used multichannel receiver and work with that. I must say though the sound quality on the lower and mid-end receivers is unacceptable.

I hope you do a thorough write up on this one, I enjoy reading about your projects.
How many toroids are you planning to use, they can get pretty big and heavy as power requirements grow.
Would you please go over correct grounding practices in detail (again)?
How do plan to handle volume control? Multichannel volume control in the amp should give the best gain structure.
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Old 6th May 2012, 10:39 AM   #10
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The star grounding looks something like this.

The choice of conductor between the boards are critical. Use the thickest possible. 4 to 5mm (!) would be great. But here I use 3mm which is fine.

Click the image to open in full size.


I learnt a lot from this post "Understanding star grounding"
understanding star grounding
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Last edited by gainphile; 6th May 2012 at 10:42 AM.
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