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Old 12th February 2010, 03:10 PM   #31
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Default Some more results.

First of all I must say that I was wrong about the bass overload on a couple of phones that I mentioned earlier. The problem was because of mixed up input cables causing very low loading on the source.

Now I have some new plots. The output impedance of the JLh phones amp. All these are measured results. The output impedance is lower than about 0.5 ohms and keeps rising with reducing frequency because of the coupling capacitor. This however doesn't cause too much of a problem as you can see from the frequency response plots with the headphone connected. I've shown the impedance plots of the headphones also. I've picked three of them. Two Philips units and one Sennheiser. That's from low impedance to high impedance. The Sennheiser has the most horrible connecting plug at the phones and I keep wanting to throw them out the window. NONE of the other headphones have any problems with connection including the Philips which has a plug at the phones. Can't believe that such an expensive headphone can have such a poor connection ! Guess they can get away because they are so big ! Did they ever do a recall ?...... like the Toyota Prius !

Since output impedance can cause ripples in the frequency response when it is loaded with varying loads like speakers or headphones, I tried a test with a 5 ohm resistor in series with the low impedance headphones ( about 40ohms). On a 1db/div plot I could hardly see any difference from the plot connected direct to the amp. The last three plots are with the headphones connected to the amp.
The plots are
1. Amp output impedance
2. Open circuit response of the amplifier
3. Philips SBC-HP800 phones impedance plot
4. Philips SHP8900 phones impedance plot
5. Sennheiser HD580 impedance plot
6. Philips SBC HP800 electrical response at the amp output.
7. Philips SHP 8900 electrical response at the amp output.
8. Sennheiser HD580 electrical response at the amp output.
Attached Images
File Type: bmp JLH-ZOUT.BMP (37.6 KB, 249 views)
File Type: bmp OPENCKFR.BMP (37.6 KB, 114 views)
File Type: bmp SBCHP800.BMP (37.6 KB, 113 views)
File Type: bmp SHP-8900.BMP (37.6 KB, 94 views)
File Type: bmp HD580-PL.BMP (37.6 KB, 127 views)
File Type: bmp SBC800R2.BMP (37.6 KB, 72 views)
File Type: bmp SHP8900R.BMP (37.6 KB, 90 views)
File Type: bmp HD580-R2.BMP (37.6 KB, 145 views)
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Old 13th February 2010, 06:09 PM   #32
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Default Capacitors and preamp use !

I used the JLH headphone amp as a preamp.
The power amp that I used sounds very good by itself. Great low end, tight bass and very clean and clear HF.

When I added the JLH amp the low end sounded different but good. HF seemed to get slightly dulled. Loss of bite in the HF !
The power amp already has a film cap at it's input. The JLH has an electrolytic cap ( 47uF/16 V Samwha ) at it's output. So out came the Samwha. Now the HF got much better and bass is still very good.

The difference between the bass ( drum notes) with the power amp by itself and via the preamp is that the drum seems to sound more powerful with the preamp.It is also better defined than without the preamp. This is not an amplitude issue. I know, I've measured the response and seen it has not changed. You can also see that at 20 Hz it's just 0.1dB down with an 8K ohm load. The difference is very audible even without an AB test.

I then changed the input cap from the 1uF yellow box cap to a 0.68uF ( all I had ) Solen cap. I expected a huge difference. I find about 10% difference ! By that I mean that it did improve the HF a bit but all else remains the same.
Some of the HF bite is back.
With the altered JLH amp the power amp sounds better though there is still a very slight loss of 'bite' in the HF. However voices seem to sound more real....tube like ? Maybe.
Try the album called "The Great Basso - Vol1 ". It's Chinese but I like the voice and great recording quality. Used as demo music by high-end shops in the Far East. As the title suggests the singer has a very bass heavy voice.

I wouldn't hesitate using this as a preamp whenever I need one. Sounds very good. However if you need to decouple the dc at the output ,you might have to use a good quality film cap. No Solen cap for the input ?.... the 1uF box is pretty decent. I must try out my low cost 1uF /250V polyester cap ! It's too late now. I have to crash out . It's almost 12.40 am .
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Old 14th February 2010, 03:56 AM   #33
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Default Time for some drastic mods.

Today is a great day. Clear sky and 26 degC ambient !
Perfect for starting a new project ! This time it will be a split supply JLH headphone amp. I expect it to sound similar to the single supply version ...but can't be sure till I try it out.
Right now the 6Volts dc of the headphone amp is biasing the input capacitor of the power amp at a much higher voltage than the usual low mV across it if it had no dc at the input ( of the power amp ). Does it help ? We'll see.
Even the split supply circuit requires an input cap which could be a good thing in general as we can never be sure if the source is dc free. With a "good" coupling cap we could remove the output coupling cap in the DAC.
That eliminates two critical capacitors in the signal path.
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Old 17th February 2010, 07:10 AM   #34
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Did some more listening tests with different input caps. From what I have I'd pick the Solen followed by the Epcos X2 caps and then the low cost film caps. I'll post a picture rather than descibe them. Top would be best and bottom would be least preffered. However I'd like to point out that even the bottom one sounds quite good by itself. If listened to with large time gaps I think it would be difficult to differentiate between them.
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Old 17th February 2010, 02:05 PM   #35
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Here are the caps. Best one on top and going down with decreasing performance. Most are quite decent though the grey X2 cap at the bottom doesn't sound as good as the rest. The blue cap is an Epcos 1uF X2 cap.
Caps in the same horizontal line are fairly similar to each other.
I'd say the Solen ( 0.68uF) has a slightly fuller sound on voice and bass ( just a wee bit ) and piano notes are just a bit more incisive.
But the difference is not 'night and day' ! The nay sayers might say that there wouldn't be a difference. Well it doesn't matter. I'll just put the cap that suits the cost of the rest of the board and which doesn't 'sound' bad. That might be the yellow box cap !
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File Type: jpg Input caps.jpg (44.6 KB, 1303 views)
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Last edited by ashok; 17th February 2010 at 02:11 PM.
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Old 24th February 2010, 02:08 AM   #36
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I just got back from an out of town trip. I see that no one appears to have tried the JLH headphone amp after I posted some results.
Well I'm on to the split supply one and tried to sim it. It seems to be good too. Setting the dc offset was a small problem though I think I have a solution. Must see if it's stable with time.
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Old 24th February 2010, 06:37 AM   #37
Mooly is online now Mooly  United Kingdom
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Quote:
Originally Posted by ashok View Post
I just got back from an out of town trip. I see that no one appears to have tried the JLH headphone amp after I posted some results.
Well I'm on to the split supply one and tried to sim it. It seems to be good too. Setting the dc offset was a small problem though I think I have a solution. Must see if it's stable with time.
It's nice having a separate forum for headphones but in a way I think it would be better in the solid state section.
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Old 24th February 2010, 01:11 PM   #38
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Hi!
I built the first JLH amplifier with +/-12V supply.
In my opinion something's wrong with it. The heat sink becomes very hot within half a minute. The heatsink is quite big, much bigger than Ashok's one on the photo as I see. It eats about 400mA, that's too much, isn't that?
I am afraid it may die if runs too much, but I measured some part of the amplifier . I attached a picture. On it there are some value that I measured using a multimeter.
What do you think? Is there any problem with it?
Thanks.
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Old 24th February 2010, 06:29 PM   #39
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Why is the base of Q5 measuring 9.6V instead of 2x Si diode drops below 12V?
Plus R10's drop, I would have expected to measure something in the high 10's.
Check if D3 D4 are oriented correctly in the real circuit?

Last edited by kenpeter; 24th February 2010 at 06:34 PM.
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Old 25th February 2010, 02:26 AM   #40
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Kenpeter is right. The voltage at the base of Q5 should be about 10.8 volts. You can change R11 to reduce the bias current in the output stage. Right now it appears to be about 0.110 amps . Additionally you must make sure the heat sink is well installed on the transistors. Silicone grease is a must on both sides of the insulating mica washer !
You must remember that in my circuit the supply is a "single" +12V supply. Voltage across the output transistors is about 6 volts each and so dissipation is "half" of what you have. You can drop the output current to just about 80 mA ( R 11 = about 6.8 ohms) to drop dissipation and you will still get enough output on a 32 ohm headphone to permanently damage your hearing ( over 110 dB spl ).

The base of Q4 also seems to be a bit high. It should be about -1.2 volts. R1 and R2 voltage drop will reduce this voltage a bit ( sim value -1.18 V ). Diodes oriented correctly ?

Do you really get 0V at the output with the components shown ? The sim shows +0.904volts ! R7 and R4 determines the output dc voltage. R7 is supposed to drop the difference of the two diode drops - (R2+R1) drops - Vbe drop of Q4. Any missmatch here will cause an error. R4 could be adjusted to get 0 V at the output. Note that R7 and R4 have the same current flowing through them (approximately). But R4 has 1.2 volts across it. So R7 will have too much across it to get 0 volts at the output. Alternatively D1/D2 should actually have one more diode in series with them! Easiest method I think is to adjust R4 and/or R7 to get less than 1 mV at the output. We will have to check how much the dc drift will be with time.
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