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Old 22nd November 2014, 08:50 AM   #1
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Default Death of Zen current adjustment

I have just built a DoZ and I am ready to adjust the current. I am not too sure I understand the proper procedure. The article from the original website is a little confusing to me on this. Rod Elliot says adjusting VR2 to get 1A quiescent current, but at the end of the article in table 3 it shows different current at different power supply voltages and load impedance.

Here is my understanding. By the way, I am planning to use 35V for power supply. I should adjust the quiescent current to about 1A without speaker load. Once the speaker is connected (8 Ohm in my case), then the peak current will be 1.5A while playing music according to the table. Then the power supply needs to be able to handle 1.5A ( or 3A for two channels).

In other words, adjust the quiescent current to 1A regardless of the power supply voltage and load impedance. Is my interpretation correct. Thanks for any help.
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Old 22nd November 2014, 11:48 AM   #2
AndrewT is offline AndrewT  Scotland
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I am going to download the DoZ article from esp and have a look.

You should have included a link to that article when you are asking for help.
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Old 22nd November 2014, 11:59 AM   #3
AndrewT is offline AndrewT  Scotland
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You can use 1.5A for Iq with +35Vcc
This limits the peak output current to 1.5Apk.
Into an 8r0 load that would require and output voltage of 12Vpk.

All the above works, if you can keep the operating temperatures reasonable. Heatsink rules.

esp's table 3 near the end is not right, he does cover himself with
Quote:
The table is not meant to be too accurate
the line applying to your 8ohms and 35Vcc cannot give 15W of maximum power.

15W into 8r0 requires an output voltage of 15.5Vpk and an output current of 1.94Apk
Clearly this is impossible with a 35V supply and an Iq = 1.5A

1.5Apk limits the maximum output power into 8r0 to 9W, not 15W
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Old 22nd November 2014, 12:00 PM   #4
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I think it's clear in the ESP article ( Death of Zen - A new Class-A power amp ) that there is no single correct setting. The project covers a range of supply voltages, the optional components to suit them and the different quiescent currents required too. A 1 amp figure could only be a nominal test set-up current to ensure all is well before hitting full power and hoping for the best. It could also be the minimum current recommended for the build in the sealed section of ESP forum but you would need to purchase PCBs to get access.

Peak output power varies with the supply voltage and the amount of quiescent current has to be sufficient to match that, according to tables 2 & 3. As you have a 35V supply, you need to have a minimum 1.5A quiescent current to drive the amplifier to 15W with an 8R resistive load. Without sufficient quiescent current, a class A amplifier just distorts badly as the output voltage is increased. Note though, real speakers fall to much lower impedance at a few hundred Hz than the label suggests so doubling that current for a 4R load would be more in line with real 8R speaker requirements. In real life though, we tend to use only trifling amounts of audio power and so sound quality may remain completely fine for personal home use where only 1~5W are all we ever use. That means simply that we may think we have a more powerful amplifier than we really do.

By now, you may have realised the simple Ohm's law relationship between current, voltage, load and hence power, as laid out in Rod's tables. With class A, the output current is drawn from the quiescent current source and this may help clarify why current must be so high with class A, rather than some small, set value as class AB biasing requires.
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Old 22nd November 2014, 12:08 PM   #5
AndrewT is offline AndrewT  Scotland
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Transformer requirement for a ClassA amplifier is usually VA = 6times to 10times the maximum ClassA power.
Two 9W channels will require an absolute minimum VA of 108.
But there is a second requirement.
The maximum continuous AC current from a transformer is VA/Vac
The maximum DC current after the capacitor input filter must be de-rated from the AC rating due to the extra heating effect of the current pulses through the rectifier.
Each manufacturer usually gives their de-rating. Expect ~70%

a 6Aac transformer can give approximately half that as a DC current after passing through the capacitor input filter. i.e. the maximum continuopus DC current will be ~ 3Adc.
Two channels of 1.5A Iq will load the 6Aac transformer to 100% of it's maximum rating, it will run hot.

I generally recommend you use a transformer at ~50% of it's maximum rating when used at a continuous duty.
i.e. for a 3A of total Iq you use a 12Aac transformer.

25Vac * 12Aac = 300VA for two channels of ClassA amplifier with a continuous DC current of 3Adc.
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Old 22nd November 2014, 01:46 PM   #6
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Andrew, take a look at the measured performance figures in table 2 of the ESP DoZ article. The nominal rating of 15W was decided when using only 1.7A quiescent current at 40V DC supply for <0.2% distortion and lets face it, nobody tries to make an ULD amplifier when "sound quality" is the primary design goal. From the circuit description, I believe this design uses a modulated bias alike the JLH 10/15 watters and hence the much lower quiescent current.

I may agree with sentiments about transformer derating but in view of the specified quiescent current, I doubt that 300VA is warranted. More to the point, the OP was asking about the significance of a 1 amp current setting, which I can't find in the article text so I guess gremlin1, that you need to talk to the ESP forum if you read it in the construction notes.
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Old 22nd November 2014, 06:27 PM   #7
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by Ian Finch View Post
..............I may agree with sentiments about transformer derating but in view of the specified quiescent current, I doubt that 300VA is warranted. More to the point, the OP was asking about the significance of a 1 amp current setting, which I can't find in the article text so I guess gremlin1, that you need to talk to the ESP forum if you read it in the construction notes.
If the amplifier with 1.5A of Iq and Vcc=35Vdc is a 15W amplifier then a two channel version has a total maximum ClassA output of 30W.
Using the standard 6times to 10times recommendation we arrive at a required VA of 180VA to 300VA.
This upper value just happens to be similar to the recommendation that my alternative method of determining how hot one wants the transformer to run.
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Old 22nd November 2014, 06:28 PM   #8
AndrewT is offline AndrewT  Scotland
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As for sliding bias, I don't know and can't simulate it and can't imagine it sufficiently that I can create a model to improve my understanding of sliding bias.
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Old 22nd November 2014, 10:27 PM   #9
sreten is offline sreten  United Kingdom
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Hi,

The DoZ and JLH are somewhat nearer a simply overbiased
pushpull class A stage than a classic single ended class A.

The former can produce twice the power for a standing
current than the latter. I think for the DoZ and JLH
its about 1.7 to 1.8 for the same standing current.

According to the article with 35V and 8R load an Iq of
1.5A will give symmetrical clipping and 15W rms.
You do not set Iq to 1A, you set it to 1.5A.

Time to look long and hard at the speakers actual
impedance curve, if you've got the heatsinking then
2A might be more appropriate for 8 ohm speakers.

rgds, sreten.

Also note if you model your speakers in something
like Basta! (Tolvan Data)* often you can reduce the
value of C3, to very good effect with some speakers.

* There is a free version that allows this modelling.
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Last edited by sreten; 22nd November 2014 at 10:36 PM.
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Old 23rd November 2014, 06:07 AM   #10
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Hi all,

Thanks for all the responses. I guess I'll start with 1.5A as Iq. However, I have a related question. I have two pairs of speakers: Large Advent ( 8 Ohm) and Small Advent ( 4 Ohm). I want to test this amplifier with both speakers for comparison. Since the speaker impedance are different, should I adjust Iq again to higher value ( maybe 3A?) when I use the 4 Ohm speakers? Or should I just simply adjust it to the higher value and leave it there to accommodate both 8 and 4 Ohm speakers?
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