The Truth Pre Amp Review

[jtwrace]

It takes a lot of elbow grease to make that turd shine!

Different customer base. Trust me.

 

[Deleted member 26571]

About the block diagram. well, I invented the thing. I did not borrow ideas of how to build it.

That is incredibly ironic, all the more so because I'm sure you don't realize it. Ed, you didn't invent anything here.

 

[Deleted member 26571]

Ed, I don't suppose you've ever tried running your preamp with the LDRs bypassed, have you? Or used the very best precision resistors you can get in place of the LDRs?

 

[MakeMineVinyl]

Ed, I don't suppose you've ever tried running your preamp with the LDRs bypassed, have you? Or used the very best precision resistors you can get in place of the LDRs?

Out of curiosity, I'd be interested in seeing the resistance curve of the LDRs - I've been working with LDRs quite a bit in the last year or so and in general they aren't terribly linear.

 

[jtwrace]

You are all aware that @tomchr measured the Tortuga LDR at one point, right?

https://neurochrome.com/pages/tortuga-audio-ldr3

 

[Deleted member 26571]

Out of curiosity, I'd be interested in seeing the resistance curve of the LDRs - I've been working with LDRs quite a bit in the last year or so and in general they aren't terribly linear.

Do you mean linearity with respect to illumination intensity, or linearity of resistance itself?

 

[MakeMineVinyl]

Do you mean linearity with respect to illumination intensity, or linearity of resistance itself?

With respect to illumination intensity.

 

[MakeMineVinyl]

You are all aware that @tomchr measured the Tortuga LDR at one point, right?

https://neurochrome.com/pages/tortuga-audio-ldr3

None of those curves are of the LDR itself vs light intensity.

 

[Deleted member 26571]

With respect to illumination intensity.

Yes, graphs I've seen are pretty severe. This explains reviews that say the volume control of That Preamp is very touchy.

I don't remember the details, but many ago I saw a circuit that used a pair of matched optocouplers and an op amp to create a linear-responding control. Basically, you adjusted a pot and the op-amp varied the master optocoupler LED current to achieve a matching resistance. It then sent the same current to the slave optocoupler which, by virtue of being matched to the master, would adjust itself to the same value.

Kinda trick, but depends on the two optocouplers having the same resistance-versus-illumination-current curve.

 

[MakeMineVinyl]

Yes, graphs I've seen are pretty severe. This explains reviews that say the volume control of That Preamp is very touchy.

I don't remember the details, but many ago I saw a circuit that used a pair of matched optocouplers and an op amp to create a linear-responding control. Basically, you adjusted a pot and the op-amp varied the master optocoupler LED current to achieve a matching resistance. It then sent the same current to the slave optocoupler which, by virtue of being matched to the master, would adjust itself to the same value.

Kinda trick, but depends on the two optocouplers having the same resistance-versus-illumination-current curve.

Sounds like a lot of trouble! I've been using a microcontroller's PWM output to control the LED and its ADC to read voltage from the LDR in a servo implementation, and that works well. In my experience, LDRs have a narrow 'somewhat predictable' range and fall off a cliff/saturate at the ends. Specimens vary pretty far from unit to unit. In any event, an LDR scheme wouldn't be my first choice for controlling volume.

 

[KSTR]

Yes, graphs I've seen are pretty severe. This explains reviews that say the volume control of That Preamp is very touchy.

I don't remember the details, but many ago I saw a circuit that used a pair of matched optocouplers and an op amp to create a linear-responding control. Basically, you adjusted a pot and the op-amp varied the master optocoupler LED current to achieve a matching resistance. It then sent the same current to the slave optocoupler which, by virtue of being matched to the master, would adjust itself to the same value.

Kinda trick, but depends on the two optocouplers having the same resistance-versus-illumination-current curve.

https://www.edn.com/use-a-photoelec...as-a-linear-voltage-controlled-potentiometer/

 

[Deleted member 26571]

Sounds like a lot of trouble! I've been using a microcontroller's PWM output to control the LED and its ADC to read voltage from the LDR in a servo implementation, and that works well. In my experience, LDRs have a narrow 'somewhat predictable' range and fall off a cliff/saturate at the ends. Specimens vary pretty far from unit to unit. In any event, an LDR scheme wouldn't be my first choice for controlling volume.

Looking at the Lightspeed documentation, it seems like they use a microcontroller to auto calibrate the LDR drivers. Complexity is relative. I'd rather not deal with a microcontroller. But if I were using microcontrollers regularly and had the toolchain set up, had development boards on hand, and was up to speed on the coding tricks, etc., of course that might be the easiest approach. I like analog circuits.

 

[Deleted member 26571]

https://www.edn.com/use-a-photoelec...as-a-linear-voltage-controlled-potentiometer/

Excellent!

 

[MakeMineVinyl]

Looking at the Lightspeed documentation, it seems like they use a microcontroller to auto calibrate the LDR drivers. Complexity is relative. I'd rather not real with a microcontroller. But if I were using microcontrollers regularly and had the toolchain set up, had development boards on hand, and was up to speed on the coding tricks, etc., of course that might be the easiest approach. I like analog circuits.

The beauty of doing it via a microcontroller is that any slope desired can be forced on the LDR, and those curves can be easily changed by user input.

 

[Deleted member 26571]

The beauty of doing it via a microcontroller is that any slope desired can be forced on the LDR, and those curves can be easily changed by user input.

I agree. The downside is I'm employed as a professional software developer and writing code feels like work to me!

Edit: I studied electrical engineering in college. I used to really enjoy coding, but like anything else, if you do it all day long, every day, the thrill wears off after a while. I can't deny the utility of microcontrollers, though.

 

[Bob from Florida]

I agree. The downside is I'm employed as a professional software developer and writing code feels like work to me!

Edit: I studied electrical engineering in college. I used to really enjoy coding, but like anything else, if you do it all day long, every day, the thrill wears off after a while. I can't deny the utility of microcontrollers, though.

The below picture and description is from the Luxman site describing the attenuator in my L-507 uX2. I don't think it uses relays - if it does they don't do any clicking. Volume changes are smooth and seamless. I particularly like the part about not changing with the passage of time. Possibly some code writing involved.

l
New LECUA*1000 computerized attenuator
The New LECUA1000 is the most important part of the pre-amplifier section and handles volume adjustment of sound signals at micro levels. A computerized attenuator, the New LECUA1000 is an integral part of the amplifier circuit and performs in the same way as the high-end C-900u LUXMAN control amplifier. The New LECUA1000 enables the volume to be adjusted smoothly and finely in 88 steps thereby minimizing the deterioration of the sound quality. In addition, a 3D layout has been introduced in the New LECUA1000 allowing the substrates to be placed in three dimensions. A direct connection between the attenuator circuit and amplification circuit is made to minimize the signal transmission route and improve efficiency. What is more, this type of design is insensitive to external vibration, eliminates any change in sound quality depending on the position and durable so that it will not decline with the passage of time.
*LECUA stands for "Luxman Electronically Controlled Ultimate Attenuator".

 

[H-713]

The below picture and description is from the Luxman site describing the attenuator in my L-507 uX2. I don't think it uses relays - if it does they don't do any clicking. Volume changes are smooth and seamless. I particularly like the part about not changing with the passage of time. Possibly some code writing involved.


View attachment 134931
l
New LECUA*1000 computerized attenuator
The New LECUA1000 is the most important part of the pre-amplifier section and handles volume adjustment of sound signals at micro levels. A computerized attenuator, the New LECUA1000 is an integral part of the amplifier circuit and performs in the same way as the high-end C-900u LUXMAN control amplifier. The New LECUA1000 enables the volume to be adjusted smoothly and finely in 88 steps thereby minimizing the deterioration of the sound quality. In addition, a 3D layout has been introduced in the New LECUA1000 allowing the substrates to be placed in three dimensions. A direct connection between the attenuator circuit and amplification circuit is made to minimize the signal transmission route and improve efficiency. What is more, this type of design is insensitive to external vibration, eliminates any change in sound quality depending on the position and durable so that it will not decline with the passage of time.
*LECUA stands for "Luxman Electronically Controlled Ultimate Attenuator".

Looks like they're using solid-state switching solutions. It works, and it's a valid solution, though I would argue that it's pretty complex for a volume control.

 

[solderdude]

I would say in complexity it is the opposite of the truth.
Only a normal potmeter followed by a buffer would be less complex than that (and perform better)

 

[raindance]

Looks like they're using solid-state switching solutions. It works, and it's a valid solution, though I would argue that it's pretty complex for a volume control.

Yeah, rather complex when you can buy the MUSES chip which sounds great (used in the Emotiva DC-1) or the TI version which Quad has been using for a while in the 99 series and the Elite series. I suspect the TI version is used in MANY applications. Quad uses solid state switching for signal routing quite successfully, such as for tone control bypass, etc, but this Luxman idea looks scary.

 

[Bob from Florida]

I would say in complexity it is the opposite of the truth.
Only a normal potmeter followed by a buffer would be less complex than that (and perform better)

Why would a potentiometer perform better than this attenuator?