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Old 30th March 2012, 06:57 PM   #12111
udailey is offline udailey  United States
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Old 1st April 2012, 12:40 PM   #12112
Pboden is offline Pboden  Norway
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Default To small transformer....?

I have a 500va/18vac toroid transformer from Antek that is supplying three F5 channels and I suspect that the transformer is too small. The transformer makes a lot of humming when its turned on.

Iam thinking of getting new ones, but are not sure what would the minimum VA for my configuration. I was thinking of a Antek AN-8420- a 800VA.
Any comments or other suggestion?
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Old 1st April 2012, 01:26 PM   #12113
oiphy is offline oiphy  Norway
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Quote:
Originally Posted by Pboden View Post
I have a 500va/18vac toroid transformer from Antek that is supplying three F5 channels and I suspect that the transformer is too small. The transformer makes a lot of humming when its turned on.

Iam thinking of getting new ones, but are not sure what would the minimum VA for my configuration. I was thinking of a Antek AN-8420- a 800VA.
Any comments or other suggestion?
you prolly got dc on your net. this will cause the torroid to make noise.
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Old 1st April 2012, 01:29 PM   #12114
Zen Mod is offline Zen Mod  Serbia
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less hum with 3-rd channel disconnected from PSU?

be sure that you don't have DC on mains line
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Old 1st April 2012, 01:53 PM   #12115
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by Pboden View Post
I have a 500va/18vac toroid transformer from Antek that is supplying three F5 channels
500VA 18+18Vac transformer has a maximum continuous AC current rating of 13.9Aac

When feeding a capacitor input filter the maximum continuous DC current rating becomes ~7Adc

Three channels of F5 draw 3.9Adc (3*1.3A of bias) that is ~56% of maximum rating.

I would expect the transformer to be OK for temperature.

It seems likely the transformer is showing symptoms of saturation due to the DC effect of distorted supply waveform.
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Old 1st April 2012, 01:55 PM   #12116
Rush is offline Rush  United States
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See Andrews answer above. He is right, as usual.
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Old 1st April 2012, 02:02 PM   #12117
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Default Caddock MP915 R5/R6 heatsink requirement

Apologies for my ignorance, but I have a question about R11 and R12.

I am building an F5 using standard diy-audio store boards, 24V rails, tech-diy parts kit plus optional Caddock MP915 and MP930 resistor kit (also tech-diy).

I understand that R1 and R2 (Caddock MP915 10R) will not require heatsinks.

I also understand that R5 and R6 (Caddock MP930 50R) will dissipate up to 5.5W (post 11134) and hence require sufficient heatsinking. I asked about placing on the main heatsink and extending the leads and Nelson replied that is OK (post 11128).

I considered placing R11 and R12 on the main heatsink as well, but am wondering if a small heatsink would also be an option. I am not sure how much these resitors will dissipate. I know that the schematic shows a 0.6V drop, so they would see a current of (I = V/R = 0.6 / 0.5 =) 1.2A (surprise, surprise, the bias current). What I do not know (shows the level of understanding, I am afraid) if this current is a constant value or depends on the load. Intuitively I would assume it to be constant, but would hate to blow up the amp on this assumption.

Assuming the current of 1.2A is correct and constant, R11 would dissipate (P = VI = 0.6 x 1.2 =) 0.7W. I am unsure of the thermal characteristics of the small heatsinks I have (they were no-name brand so no data sheet) but I would expect them to be about 20-30C/W. This should then be sufficient.

Could someone confirm this for me please?

Thanks,

Albert
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Old 1st April 2012, 02:16 PM   #12118
AndrewT is offline AndrewT  Scotland
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Some folk will listen to average levels from their speakers ~10% (-10dB) of maximum output.
Others will be happy with average levels of ~0.1% (-30dB) of maximum output.

Taking -20dB as an example that will suit many Members, then the average output power is 1/4Watt.
That equates to an average output current of 180mAac.

Don't worry about the heating effect in the feedback resistors.
It's the change in resistance due to Tj when transient currents flow that may be a concern.

Similarly the Source Resistors see the bias current as the continuous average loading. The transient heating effect can be ignored.
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Last edited by AndrewT; 1st April 2012 at 02:18 PM.
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Old 1st April 2012, 02:35 PM   #12119
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Quote:
Originally Posted by AndrewT View Post
Don't worry about the heating effect in the feedback resistors. It's the change in resistance due to Tj when transient currents flow that may be a concern.

Similarly the Source Resistors see the bias current as the continuous average loading. The transient heating effect can be ignored.
Thanks Andrew, but I am afraid your answer is aimed at a level of understanding I do not have.

I looked up the Caddock datasheet. There is no Tj mentioned. Am I correct to assume that it is the (resistive) film temperature?
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Old 1st April 2012, 02:41 PM   #12120
AndrewT is offline AndrewT  Scotland
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Originally Posted by albertNL View Post
Am I correct to assume that it is the (resistive) film temperature?
yes.
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