Roland68
Master Contributor
Here you will find some information about the circuit:
Found out the circuit of THX 789. It's nothing new
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SMSL SP400 Review (headphone amp)
- Thread starter amirm
- Start date
Found out the circuit of THX 789. It's nothing new
Hi,
Probably you know this project,
see here:
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.
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) ?
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
Master Contributor
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
Master Contributor
Then why are you even taking the SP400?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
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?
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.
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.
Hi,
I wonder, SP400 users.
As you know this amplifier run hot.
For my taste too much.
While open it I notice it is mainly it's SMPS that is really hot.
Any of you have problems with it?
I didn't have yet but I feel the temperature might damage the whole product. Normally SMPS has a very good efficiency. How much that whole consumption could be, it is possible to estimate but it is not so much to make an efficient SMPS so hot.
Thanks,
Guy
I wonder, SP400 users.
As you know this amplifier run hot.
For my taste too much.
While open it I notice it is mainly it's SMPS that is really hot.
Any of you have problems with it?
I didn't have yet but I feel the temperature might damage the whole product. Normally SMPS has a very good efficiency. How much that whole consumption could be, it is possible to estimate but it is not so much to make an efficient SMPS so hot.
Thanks,
Guy
Roland68
Master Contributor
You would have to provide an exact temperature for your SP400, because everyone experiences “hot” differently. How long can you leave your hand on the device?
Yes, I don't have a thermometer, and of course I could keep my hands on it no problem. So, believe less than 50 degrees Celsius.
It is strange to me such an opamps based device with SMPS hot.
What is more strange is that the SMPS is the hottest part.
It is strange to me such an opamps based device with SMPS hot.
What is more strange is that the SMPS is the hottest part.
I did put some audio feet under mine so there is more airflow. Mine does get „warm“ after multiple hours but I would hardly call it hot
I operated it with no cover, open box, the SMPS, surprisingly is the hottest.I did put some audio feet under mine so there is more airflow. Mine does get „warm“ after multiple hours but I would hardly call it hot
Roland68
Master Contributor
So, if you can leave your hand on it permanently, for more than 10 seconds, then we're at about 40°, rather less. This is absolutely normal and nothing to worry about at all.
These switching power supplies are very small for the output and +/- voltage, which is why the heat released is so high, but normal. The MTBF after Bellcore is probably only 180K hrs instead of 609K hrs. In industry, such power supplies run 24/7 for 5-10 years under the worst conditions at 55-60°.
The following is general on the topic and not directed specifically at you.
I'm noticing more and more "warmth" hysteria here in the forum, which is absolutely unnecessary and unfounded.
The developers know what they are doing when the devices have a stable working temperature. Temperature changes and excessive cooling, e.g. due to uncontrolled convection, are absolutely undesirable. The topic is, among other things, "Influence of temperature on the operating point". This primarily applies to diodes, transistors, etc., which are also the main components of operational amplifiers.
As long as you can place your hand on a device for more than 10 seconds, your device will not be more than 40° warm, rather less and there is no reason to worry at all, not even about its lifespan.
Even 45 -50° is not unusual for HPA, preamps and amplifiers and there is no reason to worry, there are 50 years of experience. With Class A devices you will also find temperatures above 60°, which is normal for these devices but requires a capacitor replacement after 6-10 years in continuous operation. No more expensive than one to two tanks of fuel. How often do you fill up your car?
OK, just was curious.
The DO300EX runs cool but it is a different circuit.
I am not afraid of operating points drift
I was expecting to have it warm little more than DO300EX but it is much more, but still OK as you said.
I am not always count on engineering. For example the KSA250 had a lot of problems because of its temperature
The DO300EX runs cool but it is a different circuit.
I am not afraid of operating points drift
I was expecting to have it warm little more than DO300EX but it is much more, but still OK as you said.
I am not always count on engineering. For example the KSA250 had a lot of problems because of its temperature
If worry so much about the heat on the SP400 just put a 140mm usb fan (a silent one for pc cases) in front of it. Or next to it. You can power it with a usb charger or a powerbank.
I used to do that with a old Yamaha receiver during very hot summer days .
Roland68
Master Contributor
You mean the Krell KSA250?
Sorry, but it never had a temperature problem, just a user problem.
This is a Class A power amplifier with 700 watts!!! minimum consumption, i.e. 700 watts of heat output. It has to be serviced every 6-8 years, during which not only relays, capacitors, etc. but also other parts have to be replaced and the output stage has to be readjusted. Anyone who buys something like this should know what they are getting into and adhere to the service intervals.
OK, so replacement of all these parts every 5-6 years is a good engineering?
I have a friend who had this amplifier. It was also blow output transistors while pushing Apogee.
It was always on the floor with no ventilation issues, just always has failure
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