SMSL SP400 Review (headphone amp)

[Roland68]

That I know. I can read spec sheets. I want to know why there are two of them and what is the purpose of each one.

Here you will find some information about the circuit:
Found out the circuit of THX 789. It's nothing new

 

[Guy.Cohen]

Here you will find some information about the circuit:
Found out the circuit of THX 789. It's nothing new

Hi,
Probably you know this project,
see here:

https://www.edn.com/the-g-word-how-to-get-your-audio-off-the-ground/

Among other things, this is with respect to how to make a volume control for balance signal.

one thing is strange to me, when the amplification is high the inverting leg see theoretically 0 ohm, so huge gain. I guess it does not reach that case of zero ohm practically and see that he mentioned nor to put a series resistance.

 

[Guy.Cohen]

One more, my power amplifier has SE input. The SP400 has only balance output at the back. What I am doing is taking the hot and GND into my power amplifier. Any better solution except summing actively with opamp (that has its own drawbacks along good things) ?

 

[Guy.Cohen]

Here you will find some information about the circuit:
Found out the circuit of THX 789. It's nothing new

Hi Roland,
I have a question.
In the SP400, after the R2R volume there is (I suppose) SE to Bal circuit using OPA1612, one per channel.
At that point the signal splits via 470ohm for the back XLR output but also to the 4 THX amplifiers.
I have noticed that there is a transistor connected to each output of the balancing circuit (4 BJT), where the signal splits to XLR and THX.
Do you know for what reason these transistors are there?
I was thinking about mute or biasing the opamp but I think not that and not that.

Thanks,
Guy

 

[Roland68]

Hi Roland,
I have a question.
In the SP400, after the R2R volume there is (I suppose) SE to Bal circuit using OPA1612, one per channel.
At that point the signal splits via 470ohm for the back XLR output but also to the 4 THX amplifiers.
I have noticed that there is a transistor connected to each output of the balancing circuit (4 BJT), where the signal splits to XLR and THX.
Do you know for what reason these transistors are there?
I was thinking about mute or biasing the opamp but I think not that and not that.

Thanks,
Guy

Unfortunately I can't tell you that. Since there isn't much information about the circuit and nobody knows exactly how it works, I didn't look into it any further.
Maybe you can find something in the THX patent? There are a few layout examples.

Do you also want to use the SP400 as an HPA, or just as a preamplifier?

 

[Guy.Cohen]

Unfortunately I can't tell you that. Since there isn't much information about the circuit and nobody knows exactly how it works, I didn't look into it any further.
Maybe you can find something in the THX patent? There are a few layout examples.

Do you also want to use the SP400 as an HPA, or just as a preamplifier?

Thanks.
I am using the SP400 as a preamp, pushing my SE input amplifier. I am using only pin 2 and GND, leave pin 3 on air not short to ground, although the 470ohm they put allow to short pint 3 to ground.

The 470 resistors and the BJTs at the output of the OPA1612 are between the preamp part and the THX. That the reasons I think I will not find solutions in the THX patent (or circuits).

I am dissipating a lot of heat for just using is as a preamp 3 OPA1612 I am using.

 

[Roland68]

Thanks.
I am using the SP400 as a preamp, pushing my SE input amplifier. I am using only pin 2 and GND, leave pin 3 on air not short to ground, although the 470ohm they put allow to short pint 3 to ground.

The 470 resistors and the BJTs at the output of the OPA1612 are between the preamp part and the THX. That the reasons I think I will not find solutions in the THX patent (or circuits).

I am dissipating a lot of heat for just using is as a preamp 3 OPA1612 I am using.

Then why are you even taking the SP400?
The SP400 is a pure HPA with an additional simple preamp output. For example, a Sabaj A10h for under €100 would be just as suitable.
Wouldn't a pure preamplifier make more sense?
Or, since you seem to have DIY skills, a bespoke build or conversion?

 

[Guy.Cohen]

I should bought something else. I bought the SP400 in a nice discount (~$400). I was thinking the build quality and design are very good. It is clear now the A10h fits better.

Of course I can also build something but to design PCB, to put PS, regulators, all parts, it will cost much more than SP400. We can't beat the Chinese in price/ performance.