JLH 10 Watt class A amplifier
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iperv
diyAudio Member

Join Date: Jan 2008
Location: Italy
Quote:
 Originally Posted by AndrewT No. Ibias is a DC current.
FYI:
Irms = Idc
Vrms = Vdc

Mooly
diyAudio Moderator

Join Date: Sep 2007
Quote:
 Originally Posted by iperv FYI: Irms = Idc Vrms = Vdc
RMS values don't apply to DC, only to changing quantities.

If the Ibias of the JLH69 is reduced to around 500ma (which is a static DC value) then the output can still swing almost 5 volts RMS into 8 ohms when running on a 17 volt supply. The actual peak current in the output stage will be around 900ma (falling almost to zero as the output voltage swings to ground) and with an average around 0.47A and RMS value around 0.56A
The current in the load (which is ground referenced and AC coupled) will be in the region of -/+0.9 amps peak and an RMS value of around 0.6 amp. There is no "average" current flowing in the 8 ohm load for a sinewave because its ground referenced, or more correctly the average is zero for a sinewave.

iperv
diyAudio Member

Join Date: Jan 2008
Location: Italy
Quote:
 Originally Posted by Mooly RMS values don't apply to DC, only to changing quantities.

I only said that RMS value, say 500mA, of a sinewave current is equipollent to 500mA DC current. This is (one) definition of RMS value.

Quote:
 Originally Posted by Mooly If the Ibias of the JLH69 is reduced to around 500ma (which is a static DC value) then the output can still swing almost 5 volts RMS into 8 ohms when running on a 17 volt supply.
500mA of Ibias = 500mA pk or = 500mA RMS ?
I think we are saying same things; I said that multimeters can make only RMS measurement, so when I read an Ibias = 1.75A it means an Ibias = 1.75A RMS.
Ok, RMS doesn't make sense about DC stuff, but it's a way to say that DC it's equipollent to RMS, not to peak value.
In fact max pp value of the output current signal will be = 1.75A*sqrt(2)*2.
If the reading on the display of DC current value on my digital multimeter would be equipollent to Ipk, we should have an output current signal (pp value) = 1.75A*2, or an Ibias = 1.75A / sqrt(2).
And this fact makes difference when I have to calculate the bias or say how many watts my ampli is able to express.

I think this stuff it's a little more complicated than it seems to be - and I don't understand why - but I say this beacause it seems that every person or website out there have proper definitions and workflows to calculate Ibias and Vcc.
And this appens only about audio stuff
More often than it should be, audiophiles speake about these arguments without any proper acquaintance, and this explanes - IMHO - why always there are misunderstandings about these things.

So, when you say 500mA I think (better, I hope to understand what you want to say) you mean 500mA rms (equipollent to my 1.75A rms )
17VdcBias means Vpk = (17/2) - 0.7 = 7.8Vpk circa, or 7.8/sqrt(2) = 5,15Vrms = circa 5Vrms.
This value would be the same even if the loud would be 4ohm and it is incorrelate, until a specific value, to Ibias; it is directly tied to Vcc.

Quote:
 Originally Posted by Mooly The actual peak current in the output stage will be around 900ma
So: 7.8Vpk / 8ohm = 975mA pk.
Yes, but whit an Ibias = 500mA we can only have an Ipk = 500mA*sqrt(2) = 700mA circa.
In this case we have 275mA of undervalue, that it means that after 700mA pk output current the amplifier is (heavily) clipping.

Quote:
 Originally Posted by Mooly (falling almost to zero as the output voltage swings to ground)
Ok: you are speaking of output current on the load.

Quote:
 Originally Posted by Mooly ...and with an average around 0.47A and RMS value around 0.56A.
If we are speaking of sinusoidal current it doesn't make sense to say average, in fact the average is actually zero, so I don't understand what do you want to say.
I don't understand what do you want to say with RMS value: why 0.56A?
If Ibias = 500mA DC current it means RMS output current = 500mA. No doubt at all on this point.

Quote:
 Originally Posted by Mooly The current in the load (which is ground referenced and AC coupled) will be in the region of -/+0.9 amps peak and an RMS value of around 0.6 amp.
Again, I agree with +/-0.9A (but whit the limitation of clipping), but I don't understand what do you mean with RMS value.
(EDIT: ok, you mean RMS value of +/- 0.9A. But you have to consider that is impossible to correctly drive a load of 8ohm with a supply of 17Vdc at full power without clipping if Ibias is set to 500mA; amplifier needs an Ibias around 0.6A, that is the value that you call RMS).

Quote:
 Originally Posted by Mooly There is no "average" current flowing in the 8 ohm load for a sinewave because its ground referenced, or more correctly the average is zero for a sinewave.
In fact.

Last edited by iperv; 22nd March 2013 at 12:27 PM.

 22nd March 2013, 12:39 PM #2714 Mooly   diyAudio Moderator     Join Date: Sep 2007 Well we are talking about a very specific circuit here and you have to take the bootstrap part of the circuit into account as well because that alters the dynamics of it all completely. Remove the bootstrap (which changes nothing DC wise) and you lose a lot of "positive" drive capability. Ibias is the steady state quiescent current that you set to flow in the output stage. For this circuit (in its standard form) the output can deliver approximately Ibias * 2 for positive excursions in output. Negative excursions (the bottom half of the sinewave if you will) have no such restriction. So that does give asymmetric clipping, but for an 8 ohm loading it will deliver just short of 5 volts RMS.
 24th March 2013, 03:28 PM #2715 vega24   diyAudio Member   Join Date: Mar 2013 Location: Nimes Hello, I just finish my JLH HOOD 1969 10W with 2sc5200 but the result is not very fine, I have a power supply 16 to 28V, normally this item works with +27V, but if I use +27V, the sound is very horrible with pic.. I find a good voltage, +16V, but for me it's not normal voltage.. anybody can help me ? thanks
 7th May 2013, 04:06 AM #2716 MKHunt diyAudio Member   Join Date: Jan 2013 Location: SLC, Utah Just wanted to share my JLH progress with other JLHers. I am using the 2SC3280 transistors and I'm not super sure what power supply wattage to shoot for. I supposedly have a 200W SMPS on the way from Connexelectronic but I fear that may be too large to power the stereo pair. And the preamp that I built to feed the JLH
 7th May 2013, 06:51 AM #2717 Mooly   diyAudio Moderator     Join Date: Sep 2007 The amp will draw whatever it is constructed to draw. You can't have to large a capacity supply. If the amp pulls 2 ? amps at 27 volts then you need at least a 54 watt supply. Double that for two channel. Add a safety margin too. say 140 watt supply needed for that example for two channels. Is the 200 watt SMPS rated 200 watts output or is that what its nominal power consumption from the mains is. There's a big difference. 200 watts mains consumption will be nothing like what it can deliver to a load.
 7th May 2013, 07:10 AM #2718 mindutis   diyAudio Member     Join Date: Apr 2008 Location: Sweden The heatsinks will be too small for this much (54W per channel) power dissipation. __________________ My DIY amplifier
MKHunt
diyAudio Member

Join Date: Jan 2013
Location: SLC, Utah
Quote:
 Originally Posted by Mooly The amp will draw whatever it is constructed to draw. You can't have to large a capacity supply. If the amp pulls 2 ? amps at 27 volts then you need at least a 54 watt supply. Double that for two channel. Add a safety margin too. say 140 watt supply needed for that example for two channels. Is the 200 watt SMPS rated 200 watts output or is that what its nominal power consumption from the mains is. There's a big difference. 200 watts mains consumption will be nothing like what it can deliver to a load.
SMPS is rated for 200W of output. Good to know the amp will not draw more than it is constructed to. Since it uses the SiliconRay 10W kit (2N3055 swapped for 2SC3280 for convenience of wall mounting, datasheets paint them as nearly identical). SO with an Iq of ~2A and a rail voltage of 22V I could get away with a 120W PSU (2*22*2 = 88W)? 88W in for 20W out (total) is what, 22% efficiency?

The real problem is where to find a very compact center-tapped PSU. I placed that order with Connexelectronic on April 8 and have heard literally nothing from them. I could have ordered twice from overseas in this time frame.

Quote:
 Originally Posted by mindutis The heatsinks will be too small for this much (54W per channel) power dissipation.
Oh lawd yes. 54W of heat load is like a desktop processor.

Last edited by MKHunt; 7th May 2013 at 02:53 PM. Reason: SiliconRay JLH is 1996 v not 1969

thoglette
diyAudio Member

Join Date: Dec 2008
Quote:
 Originally Posted by MKHunt I supposedly have a 200W SMPS on the way from Connexelectronic but I fear that may be too large to power the stereo pair.
If I was using a SMPS (switch mode power supply) I'd not be mounting it in the same chassis as the amplifer. And I'd be putting a pile of noise reduction (staged sets of pi filters) between the amp and the SMPS. Given this is Class-A you can get very serious about it: F3db for the PS is below 1hz.

But remember that there is a reason Jean Hiraga used batteries.

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