bias supply problems and thoughts

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Disabled_shredder
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bias supply problems and thoughts

Post by Disabled_shredder »

so im building a completely custom micro amp, the details are not important for this discussion i believe. i have a hammond 269ax 250vct transformer. i used the 90 kelly trainwreck schematic as a reference for my bias supply all values are exactly as specified in the schematic. the problem is before i hook up my 20k pot the voltage is -109 with the pot hooked up it will swing between -5v and 0v. does anyone have advice on how to get my -v up or have a schem for a bias supply that doesnt use a 50v secondary?
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Dr-Joned
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Re: bias supply problems and thoughts

Post by Dr-Joned »

You are going from an almost 600vct to 250vct transformer. You will need to change your dropping resistor (220k) to a lower value such as 100k or lower. This should boost up the bias output.
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Disabled_shredder
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Re: bias supply problems and thoughts

Post by Disabled_shredder »

i was starting to think that. thank you so much ill give that a try and post results.
Disabled_shredder
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Re: bias supply problems and thoughts

Post by Disabled_shredder »

so some thoughts on this issue. im down to 9.1k at -33v dc. im gonna go lower. but in change after the diode the voltage is -190 way more than my 100v rated caps. guess its a trade off! also im using 2 watt metal oxides do you think i can get away with 1 watt metal film or metal oxide?
John_P_WI
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Re: bias supply problems and thoughts

Post by John_P_WI »

Without a drawing I don't know exactly what you are doing here, but it is possible to put a resistor between the "bottom leg" of the pot and ground, effectively moving the pot "up" the voltage ladder. In this case, more negative voltage. It is best not to change the resistor values wildly as it impacts the time constants of the circuit.
Disabled_shredder
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Re: bias supply problems and thoughts

Post by Disabled_shredder »

problem fixed! lowered the 220k resistor down to a 4.7 or 2.2 k and upped the 10uf/100 caps to 10/250 now i have an available -46 or so volts! thanks for the help dr-joned
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Phil_S
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Re: bias supply problems and thoughts

Post by Phil_S »

Good that you have the desired result, but it isn't necessary to guess at these things. If you don't have the direct knowledge to make the calculations, you can d/l Duncan's PSU, which a great simulator. It will allow you to fiddle with the various components and show reasonably accurate predictions. This, IMO is something everyone can get and learn how to use.

I may not be right about this, but have some lingering concerns about using a dropping R as low as 5K as part of its function is decoupling. The higher value does a better job of decoupling. Without doing any calculations, my gut tells me to solve this particular problem with a dropping resistor of at least 47K and then work on the voltage divider after the diode rectifier to control the voltage.

You can model all of this in the PSU so you don't have much trial and error work. If you are going to acquire the amp building addiction, learning how is always worth the effort...the difference between being given a fish and learning to fish, if you know what I mean.
Disabled_shredder
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Re: bias supply problems and thoughts

Post by Disabled_shredder »

where can i download the psu designer i tried from the sight but its just a word file getting saved to my cpu. i really think it would be helpful
Disabled_shredder
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Re: bias supply problems and thoughts

Post by Disabled_shredder »

i have a mac and i think it only works with windows
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Colossal
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Re: bias supply problems and thoughts

Post by Colossal »

Disabled_shredder wrote:i have a mac and i think it only works with windows
I run Sun's VirtualBox (freeware) running Wind'ohs 95 on a Mac so I can run some PC only utilities. Works perfectly.
Disabled_shredder
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Re: bias supply problems and thoughts

Post by Disabled_shredder »

I was able to download the program but it has no option to set up bias supply ill post a schematic if possible and maybe we can find a better way to do this though ive got the neg voltage i desire so we will have to see if it gives any bad results down the line
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Phil_S
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Re: bias supply problems and thoughts

Post by Phil_S »

If you post a schematic and your desired voltage, I'll be happy to model it for you this time. You just have to experiment with it for a while until you figure out how to build the circuit in the PSU. The data input requires the DCR and actual voltage of the high voltage secondary winding, too. Once you have some screen shots of a model, you should be better able to use it next time.
Disabled_shredder
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Re: bias supply problems and thoughts

Post by Disabled_shredder »

this is the best i could do by using the 4.7k resistor i got the desired -dc voltage but someone earlier in the messages said it might have an adverse affect lowering the resistor value that much origionally it was supposed to be a 220k. i am only concerned with the bias supply also. the main psu is working like a charm
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John_P_WI
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Re: bias supply problems and thoughts

Post by John_P_WI »

Swapping places of r8 and r9 would give more negatve voltage without lowering the values the wsy you did and maintain the time constants.
Disabled_shredder
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Location: South Carolina

Re: bias supply problems and thoughts

Post by Disabled_shredder »

that doesnt make sense. so run the 5ok pot with the 47k resistor were? on pin 1 going to ground?
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