JohnAudioTech, per my earlier comment about the time delay circuit in this amplifier, I have figured out the 'mystery' transistor circuit. This is a feature to prevent damage to the headphone's internal speakers if the amplifier tries to output a signal with a significant DC component (or a DC-only signal). This could happen because this amp is entirely DC coupled (no AC coupling capacitors). If either or both channels' amplifier stages (prior to the relay contacts) have a DC component of more than about 0.8V, the relay will be de-engergized, thereby disconnecting the headphone jack from the amplifier stages. This state will continue until a few seconds after the DC component has decreased to a lower voltage. It is nice to see that the circuit's designer thought of this.
Glad that U talked about the independence issue. I have a set of Koss studio pro headphones that have around 250 to 300 ohms. and cannot find a decent preamp to drive them. It would be great to see a future vid on this topic perhaps a diy headphone amp or a tube preamp diy.
JohnAudioTech, I have purchased the revised version of this kit, the HA-PRO2. Seems to be the same as what you reviewed, except the alternate output terminal block (between the main headphone jack and the volume potentiometer) has been replaced with an 1/8" headphone jack, so it has both 1/4" and 1/8" jacks. I have reverse engineered the circuit, and am drawing up a schematic; seems a lot of people are asking for the schematic, and it is unobtainium, so I will try to put mine online somewhere downloadable. In studying the circuit (I have not actually built mine yet), I see that there is indeed a time delay of nearly one second after turning the power on before the relay closes its contacts, connecting the circuit outputs to the jacks. But I see no time delay on turning the power off. You mentioned this, but did you actually observe a power-off time delay? I think it just immediately disconnects the jacks from the amplifier circuit when the power is removed....indeed, there is a diode in the circuit to make sure it de-energizes the relay quickly. I did notice one mystery in the circuit; while there are two transistors in a Darlington pair to control the relay coil, there is another pair of transistors that I cannot figure out the purpose of in the circuit. From your video, it looks like yours has the same circuit. There is a 220uF capacitor and those two extra transistors and a pair of 10k Ohm resistors that comprise the circuit in question. The way the transistors are interconnected is odd, and they seem to be driven from the Left and Right outputs (BEFORE THE RELAY CONTACTS) via the two 10k resistors. I am not great with bipolar transistor theory, but as near as I can figure, the circuit monitors both channels pre-relay and somehow this influences the relay time delay duration. I thought perhaps if there is currently no audio signal coming through the amp, i.e; if it is amplifying silence, then the relay time delay might be shortened a bit, but if there is active audio, then the circuit detects it and lengthens the time delay. Or vice versa, I am not sure of my analysis. Since I have not assembled the PCB yet (I am waiting on my torroid power transformer and Rean RCA jacks, which should perfectly fit the aluminum case that is made for this kit), I rigged up the same mystery circuit on a solderless breadboard, powered from my bench supply, and instead of an amplifier output I used the output of my signal generator. I confirmed the power-up versus power-off time delays, or lack thereof. But no matter what I tried, I could not get the mystery circuit to have any effect on the time delay, and I don't see any other effect it COULD have. Any ideas? Also, with my HA-PRO2 kit, there were also two extra components that do not go on the circuit board; a 100 Ohm 1W film resistor and a 1nF (0.001uF) 1kV disk capacitor. A tiny schematic on the circuit board silkscreen suggests that these be connected as a series RC snubber network wired in parallel with the power switch contacts. Those values do seem appropriate for an application like this. I just wonder if your kit came with the same thing.
Great show and board John, if people only have a fully AC transformer type wall wart adapter, with just a single (no center tap), output - you can easily reconfigure the supply input circuitry for split rail voltage doubling. For about 16 Ohm (low Z), type headphones - an about 18 Ohm 1 Watt dropper resistor in series would help match. Nice to have had a 3.5 mm output socket, but there are adapters that can be used. PS - I hope you dad is doing well, best wishes Michael.
I've never heard of op-amps oscillating while the supply voltage drains. In some of my tube circuits I've noticed some weird oscillation when I turn off the power, when the voltage gets to a certain point the circuit makes some wierd noises.
Out of topic question. I found a baxandall tone control ckt and a preamp ckt. Which should come first? Preamp+tone_control+amplifier? Or tone_control+preamp+amplifier?
11.10 $ US (free shipping worldwide) I think I'll order one. My son bought high end headphones (100 ohm) which plays very weak connected to a smartphone. Obviously, one has to add a box and a transformer and maybe a decent looking volume knob?
You have found it? Can you provide a link or some keywords? I tried ebay or aliexpress and no luck :\ EDIT: as always, when you start bothering peoples, you found it... (on Aliexpress) goo.gl/Z3oRDB
1959Berre. Radio shack online (they still exist, kind of) has some cheap halfway decent looking knobs for sale. And some non-inductive 8 ohm 20 watt resisters FWIW.
eBay sells an aluminum case designed to fit this kit. It comes with all the hardware, an IEC power cord connector, plus a nice looking volume knob that fits the kit's potentiometer, and a power switch. Price is about the same as the kit itself. You still need to source a power transformer, an IEC power cord, and connectors for the inputs. The case seems to be designed for a pair of Neutrik-Rean model NYS367-0 (black) or -2 (red) or -9 (white) RCA jacks, which are nice looking and good quality and readily available. I think a Telema model 0015P2-2-012 torroid power transformer would be a very good fit, but I have not checked sources and availability yet.
Thanks for your reply. I'm quite new to audio so your channel is really helpful and any knowledge I can glean is truly appreciated! It's mainly through your channel that I have discovered you can get good quality audio for a really quite reasonable outlay. Thanks again Davy
You did mention the output resistors were 10ohm.Wouldn't it be wrong to use this with headphones having less then 80 ohm impedance.It would effect the headphone's frequency response because of high output impedance?
I'm amazed that it's so flat and that you keep the same tone of voice no matter what's happening your so flat and void of emotion that it's almost confusing to the listener
JohnAudioTech, per my earlier comment about the time delay circuit in this amplifier, I have figured out the 'mystery' transistor circuit. This is a feature to prevent damage to the headphone's internal speakers if the amplifier tries to output a signal with a significant DC component (or a DC-only signal). This could happen because this amp is entirely DC coupled (no AC coupling capacitors). If either or both channels' amplifier stages (prior to the relay contacts) have a DC component of more than about 0.8V, the relay will be de-engergized, thereby disconnecting the headphone jack from the amplifier stages. This state will continue until a few seconds after the DC component has decreased to a lower voltage. It is nice to see that the circuit's designer thought of this.
Glad that U talked about the independence issue. I have a set of Koss studio pro headphones that have around 250 to 300 ohms. and cannot find a decent preamp to drive them. It would be great to see a future vid on this topic perhaps a diy headphone amp or a tube preamp diy.
When you asked to spot the error I didn't notice the terminal block I was complaining about the regulators so close to those capacitors.
JohnAudioTech, I have purchased the revised version of this kit, the HA-PRO2. Seems to be the same as what you reviewed, except the alternate output terminal block (between the main headphone jack and the volume potentiometer) has been replaced with an 1/8" headphone jack, so it has both 1/4" and 1/8" jacks.
I have reverse engineered the circuit, and am drawing up a schematic; seems a lot of people are asking for the schematic, and it is unobtainium, so I will try to put mine online somewhere downloadable.
In studying the circuit (I have not actually built mine yet), I see that there is indeed a time delay of nearly one second after turning the power on before the relay closes its contacts, connecting the circuit outputs to the jacks. But I see no time delay on turning the power off. You mentioned this, but did you actually observe a power-off time delay? I think it just immediately disconnects the jacks from the amplifier circuit when the power is removed....indeed, there is a diode in the circuit to make sure it de-energizes the relay quickly.
I did notice one mystery in the circuit; while there are two transistors in a Darlington pair to control the relay coil, there is another pair of transistors that I cannot figure out the purpose of in the circuit. From your video, it looks like yours has the same circuit. There is a 220uF capacitor and those two extra transistors and a pair of 10k Ohm resistors that comprise the circuit in question. The way the transistors are interconnected is odd, and they seem to be driven from the Left and Right outputs (BEFORE THE RELAY CONTACTS) via the two 10k resistors. I am not great with bipolar transistor theory, but as near as I can figure, the circuit monitors both channels pre-relay and somehow this influences the relay time delay duration. I thought perhaps if there is currently no audio signal coming through the amp, i.e; if it is amplifying silence, then the relay time delay might be shortened a bit, but if there is active audio, then the circuit detects it and lengthens the time delay. Or vice versa, I am not sure of my analysis.
Since I have not assembled the PCB yet (I am waiting on my torroid power transformer and Rean RCA jacks, which should perfectly fit the aluminum case that is made for this kit), I rigged up the same mystery circuit on a solderless breadboard, powered from my bench supply, and instead of an amplifier output I used the output of my signal generator. I confirmed the power-up versus power-off time delays, or lack thereof. But no matter what I tried, I could not get the mystery circuit to have any effect on the time delay, and I don't see any other effect it COULD have. Any ideas?
Also, with my HA-PRO2 kit, there were also two extra components that do not go on the circuit board; a 100 Ohm 1W film resistor and a 1nF (0.001uF) 1kV disk capacitor. A tiny schematic on the circuit board silkscreen suggests that these be connected as a series RC snubber network wired in parallel with the power switch contacts. Those values do seem appropriate for an application like this. I just wonder if your kit came with the same thing.
Great. Thank you for the run testing. Sick of kit builds w/o it. Why watch? Do more.
Great show and board John, if people only have a fully AC transformer type wall wart adapter, with just a single (no center tap), output - you can easily reconfigure the supply input circuitry for split rail voltage doubling. For about 16 Ohm (low Z), type headphones - an about 18 Ohm 1 Watt dropper resistor in series would help match. Nice to have had a 3.5 mm output socket, but there are adapters that can be used. PS - I hope you dad is doing well, best wishes Michael.
nice headphone amplifier without coupling capacitors, that might be the secret behind perfectly flat audio response.
Proper value caps will give perfectly flat response as well, just not down to DC, but audio doesn't need DC response.
I've never heard of op-amps oscillating while the supply voltage drains. In some of my tube circuits I've noticed some weird oscillation when I turn off the power, when the voltage gets to a certain point the circuit makes some wierd noises.
If for some reason one supply rail goes to 0 before the other, I've heard them squeal like a stuck pig.
Please make a board review on Bluetooth audio kit. Thanks
Very neat assembly, very informative video, thanks.
Nice work. About the stereo channels blend though. I'm so used to high stereo separation that it gives me euphoria 🙂
Why should use so many 4580? One 4556 would be enough, doesn't it?
Cheers from Indonesia
Out of topic question. I found a baxandall tone control ckt and a preamp ckt. Which should come first? Preamp+tone_control+amplifier? Or tone_control+preamp+amplifier?
It depends on your circuit design. So the tone control filters are not affected by input impedance, I'd "buffer" them with a preamp.
JohnAudioTech thanks!
11.10 $ US (free shipping worldwide) I think I'll order one. My son bought high end headphones (100 ohm) which plays very weak connected to a smartphone. Obviously, one has to add a box and a transformer and maybe a decent looking volume knob?
You have found it? Can you provide a link or some keywords? I tried ebay or aliexpress and no luck :\
EDIT: as always, when you start bothering peoples, you found it... (on Aliexpress) goo.gl/Z3oRDB
1959Berre. Radio shack online (they still exist, kind of) has some cheap halfway decent looking knobs for sale. And some non-inductive 8 ohm 20 watt resisters FWIW.
eBay sells an aluminum case designed to fit this kit. It comes with all the hardware, an IEC power cord connector, plus a nice looking volume knob that fits the kit's potentiometer, and a power switch. Price is about the same as the kit itself. You still need to source a power transformer, an IEC power cord, and connectors for the inputs. The case seems to be designed for a pair of Neutrik-Rean model NYS367-0 (black) or -2 (red) or -9 (white) RCA jacks, which are nice looking and good quality and readily available. I think a Telema model 0015P2-2-012 torroid power transformer would be a very good fit, but I have not checked sources and availability yet.
JRC 4556 {(used in RA1, O2, cmoy) are better in driving headphones than jrc4580.
Are most preamp boards suitable for headphone use and vice versa? Is it just a reduced level of amplification or is there more to it?
A standard preamp is not designed to drive the lower impedance of headphones.
Thanks for your reply. I'm quite new to audio so your channel is really helpful and any knowledge I can glean is truly appreciated! It's mainly through your channel that I have discovered you can get good quality audio for a really quite reasonable outlay. Thanks again
Davy
gartmorn - If you’re new, you’re in the right spot. Stay w/ the channel. Learn everything you can on impedance. Golden thread.
Nice build
You did mention the output resistors were 10ohm.Wouldn't it be wrong to use this with headphones having less then 80 ohm impedance.It would effect the headphone's frequency response because of high output impedance?
Nice!!
A bit pointless review as you don't and we don't know who makes the kit
nice! :D
I'm amazed that it's so flat and that you keep the same tone of voice no matter what's happening your so flat and void of emotion that it's almost confusing to the listener