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Old 16th February 2004, 10:52 AM   #1
digi01 is offline digi01  China
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Default my opa548 headclone for AKG-K1000

I like P2P wiring
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Old 16th February 2004, 05:44 PM   #2
sek is offline sek  Germany
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For a K-1000?
How does it sound? And how does the sound compare to the specs in the datasheet, especially the THD+N curve?

Looks good
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Old 16th February 2004, 11:25 PM   #3
digi01 is offline digi01  China
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i heven't finish it,I am considering the overall arrangement and structure now.The type of my opa548 package is DDPK,so It will be difficult to install.
on the another hand,K1000 is a very special headphone,need more large power.

these is some specifications of K1000:
Impedance: 120 Ohms
Sensitivity: 74dB for 1mW free field
Frequency Response: 30Hz to 25kHz
Power Requirement : 100mW for 1Pa
Distortion at 400mW: around 100dB
- 2nd harmonic <0.5% 200Hz to 2kHz
<1.0% 100Hz to 200Hz
Power Rating: 1000mW (test tone DIN 45582) corresponds to approx 104dB

So,I think the range of K1000 suitable powers is between 100 and 1000mws.Pmax=10lg(1000/1)=30dBm.
If the source has a 0dB output such as the portable cd player,Then the amp needs 30-10lg(600/120)=23dB to get the MAX 104dB SPL.
I am using BB's opa548 making this amp now.The OPA548 is a high-voltage/high-current op ampŁ¬and it can driving a wide variety of loads. Because sensitivity of K1000 is relatively low,and Its power range is relatively wide(1mw~1000mw).So a single gain stage will be not good.i have to do some more thinking after system matching tests...
now,I set my k1000 amp in this way,a +10dB pre-amp and a +26dB output driver amp.The pre-amp can be bypass.I wanted to know if anyone had any thoughts on the design

thanks a lot.
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Old 16th February 2004, 11:29 PM   #4
digi01 is offline digi01  China
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detail pic,the pre-amp is a opa2604.
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Old 17th February 2004, 10:43 AM   #5
sek is offline sek  Germany
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I was asking the above question because I've got OPA541 and OPA549 laying around, I wanted to use them for an active speaker system.

I pretty much like your amp. Although I usually tend to dislike P2P wiring, I find your's actually beautiful. I guess it's because of the copper.

Let me just calculate that through. The K1000 has a nominal impedance of 120R. Do you have (or can you measure) an accurate impedance curve, btw.?

Given a maximum desired output of 1000mW, the good old equation P=U*U/R lets me think of a voltage between 10V and 11V to achive 1000mW into 120R.

You're planning to implement an overall gain of +36dB, including pre- and power stage. Man, what kind of sources do you use?

Quote:
Pmax=10lg(1000/1)=30dBm
The power level of 0dBm is defined to be a power of 1mW (into a 600R load, according to definition). A voltage level of 0dBV is defined to be 1Vrms. That means, calculating a resulting power level out of a given voltage level involves your (K1000's) input impedance.

Calculating is done with the equations (in the voltage domain) dB = 20 log (V1/V2) and (in the power domain) dB = 10 log (P1/P2).

Also, have a look at a handy tool here: http://www.sengpielaudio.com/calculator-db-volt.htm

Quote:
If the source has a 0dB output such as the portable cd player
Well, 0dB in respect to what? As a Bel scale always has to be referenced, it's important to be sure of the correct calculation here. The K1000 is just too expensive to expose it to an amp with a wrong gain structure...

Considering a rough 0dBV as the output of your cd player, your 36dB of gain (voltage gain of 63, according to the above equation) would result in an opa's output of 63V. Assuming a flat 120R impedance curve, this would result in an output power of P = 63V*63V/120R = 33W.

Your cd player will probably easily deliver you an output voltage of 1Vrms (and more). Who's making the mistake? Me, I hope.

Quote:
I think the range of K1000 suitable powers is between 100 and 1000mws.
As you probably want to use your head-amp independently of your portable cd player (not glued together, that is ), you should stick with an input sensitivity that fits some standards. Or one that fits your source(s), if you want to be individual.

An output power of 1000mW with a standard source of -10dBV (0.316V, older HiFi-standard, according to DIN and ISO) would require an overall gain of a rough 30dB (voltage amplification factor of nearly 32).

OTOH, an output power of 1000mW with a standard source of +4dBu (1.224V, professional standard, according to IEEE, ITU and whatever) would require an overall gain of a rough 18dB (voltage amplification factor of 8).

Quote:
I wanted to know if anyone had any thoughts on the design
What? More?

Well, just one: Have you ever wondered why next to nobody's actually using a power amp for headphone amplification?

BTW: 1000mW is not extraordinarily much power for a headphone. The BUF634, for example, can deliver a rough 250mA, works with a +/-15V supply and integrates perfectly with most opamps as a (voltage) gain stage. That makes the BUF634 deliver (say, at 12V output voltage capability) 1.200mW into 120R. It's too low for the K1000 to have headroom left (because of the relatively high 120R), but a 50W power chip-amp as the next bigger step? Think about it...
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Old 17th February 2004, 01:00 PM   #6
macboy is offline macboy  Canada
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I agree with Sek's assessment: way too much gain overall. The standard line level is 2 V p-p or 0.7 Vrms. Caculate the RMS voltage that you need for "full" power with your headphones (keeping in mind that it will probably be ridiculously loud) and determine gain from that. P=(V^2)/R

I would strongly recommend using a much lower gain than 26 dB for the amplifier (OPA548) stage. This amp has a much lower gain bandwidth product than any of National's chip amps. You said you'd use a gain of 26 dB, which is a voltage gain of 20. The gain-bandwidth product is 1 MHz, so you have a bandwidth (-3 dB) of 50 kHz. That is close enough to the audible region to affect it. Your response will be down 1 or 2 dB at 20 kHz. I very strongly recommend running a much lower voltage gain (like 5) and getting the rest of the required gain from a pre-amp stage. Also, try the chip in inverting mode. It seems to perform well that way (see the overshoot vs. load capacitance chart).
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Old 20th February 2004, 12:59 AM   #7
digi01 is offline digi01  China
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thanks sek,you are right.
Let me just show my calculation again.See where to make mistakes.
The K1000 has 74dB SPL for 1mW,104dB SPL for 1000mW.Calculating the required gain from 1mw to 1000mw is done with the equations(in the power domain) dB = 10 log (P1/P2).
that is 10log(1000mw/1mw)=30dBm.
It is incorrect that I want to calculate in this way..that my gain set up too high.
I think me mixing up voltage and power levels.

thanks macboy for you point out the characteristic of opa548's gain bandwidth product, let me think deeply again.

this is my revised sch version,the opa548's relatively high input offset is cancelled by use of an opa2604 as an input stage. The gain is set at 10(Vout/Vin) overall by R3 and R4. the PSU uses a series of batteries.
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Old 20th February 2004, 01:12 AM   #8
digi01 is offline digi01  China
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Old 20th February 2004, 01:49 AM   #9
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Default seems like an interesting project

after all the gc's I've made

I see you have some exotic components in the circuit

could you give me some details about them?


Thanks


J-P
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Old 20th February 2004, 02:05 AM   #10
digi01 is offline digi01  China
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Hi uvodee,I have not completed this mini GC yet, just fix the whole structure now.The present question is how to set up the gain and topological of PRE-op and 548 stage.
If you are interested in my style, can see some former works of mine.

my site
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