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Tear down of Massdrop THX AAA 789 Headphone Amp

Michael Kelly

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AndrewMason

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Hi @amirm. The THX AAA implementation of feed forward plus feedback is unique and heavily patented (all patents unexpired).

Its incorrect to assume it isn't covered by current patents.

Re DC coupling: to achieve, one could short each of the 4 small electrolytics with small wire jumpers. This would void the warranty. But so long as the DAC source is well behaved, this isn't dangerous for the electronics.
 

Michael Kelly

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Hi @amirm. The THX AAA implementation of feed forward plus feedback is unique and heavily patented (all patents unexpired).

Its incorrect to assume it isn't covered by current patents.

Re DC coupling: to achieve, one could short each of the 4 small electrolytics with small wire jumpers. This would void the warranty. But so long as the DAC source is well behaved, this isn't dangerous for the electronics.

I would suggest to measure the DC offset on each side of the caps first. There may be dc levels internally that explain why they used capacitive coupling. I am guessing that the TLC062 is doing DC offset adjustment, but it's not clear from their patent if that is possible with the feed-forward.

Frankly this is kinda fun to see what we can understand from the outside!
 

maverickronin

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Just a thought but would electronic volume control be easier to implement on balanced circuit versus analogue potentiometers ?

It depends how you do it. Volume control chips would be cheaper than good pots but I don't think any of the off the shelf chips have performance up to the standards of the rest of the circuit since they introduce their own distortion.

To get performance up to the level of its amp section you'd need a relay driven resistor ladder like Benchmark's HPA which can get damned expensive if you want decently small step sizes.
 

AndrewMason

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Please tell me I can mount large caps there, and make it into a 150W 8ohm speaker amp!
If only it were so easy :)
I always include a few PCB placeholders to give flexibility. But please don't populate those caps now - it'd require re-doing some stress calculations elsewhere and possibly adjusting some other elements... likely for no benefit.
 

tomchr

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tomchr

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I guess it's actually easier/better to do attenuation in single-ended?
Indeed it is.

Also note that there isn't really any technical advantage of using fully differential/balanced circuits at these distortion levels. The idea that "balanced is better" stems from the tube days where balanced circuits were sometimes employed to reduce distortion. In theory this works well. In practice it's usually better to avoid generating the distortion in the first place. Often cancellation circuits actually make the performance worse. Also note that a balanced circuit cancels the even order harmonics - those that most find pleasing. The odd order harmonics aren't canceled.

From a business perspective, balanced circuits become a non-starter pretty quickly in low THD designs. Somehow the idea of using twice the number of parts to degrade performance only to get a bullet point on the marketing copy doesn't seem to go over well. :)

Tom
 

LeoL

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Indeed it is.

Also note that there isn't really any technical advantage of using fully differential/balanced circuits at these distortion levels. The idea that "balanced is better" stems from the tube days where balanced circuits were sometimes employed to reduce distortion. In theory this works well. In practice it's usually better to avoid generating the distortion in the first place. Often cancellation circuits actually make the performance worse. Also note that a balanced circuit cancels the even order harmonics - those that most find pleasing. The odd order harmonics aren't canceled.

From a business perspective, balanced circuits become a non-starter pretty quickly in low THD designs. Somehow the idea of using twice the number of parts to degrade performance only to get a bullet point on the marketing copy doesn't seem to go over well. :)

Tom
Good point, Tom. Also, while the benefits of internally processing the signal fully balanced are tenuous, I believe that there are benefits to differential transmission of signals between gear in noisy environments and/or if the signal has long distances to travel.
 

tomchr

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Good point, Tom. Also, while the benefits of internally processing the signal fully balanced are tenuous, I believe that there are benefits to differential transmission of signals between gear in noisy environments and/or if the signal has long distances to travel.
I agree wholeheartedly on the differential signalling between pieces of equipment. Within the boxes, signal integrity can be maintained by good circuit design and layout.

Tom
 

Silou

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Are there any informations about the upcoming measurements of the Loxjie P20? I own one myself and I think it is a really cool amp.

best regards
 

Veri

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Are there any informations about the upcoming measurements of the Loxjie P20? I own one myself and I think it is a really cool amp.

best regards

It is in the backlog, can take some time you will have to be patient :)
 
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