G'day Guys,
I'm brewing up a tube preamp and my question regards using a single transformer with both a HV and a heater secondary vs using separate transformers for HV and heaters.
I'm not going to claim I understand all of the issues involved. However from my research on the matter it appears to be that multiple transformers is the better solution.
This appears to be for 3 reasons:
1) Interwinding capacitance
2) Magnetic coupling of noise between the secondaries
3) Cost: I asked our local transformer winder for a quote for a single EI transformer and was quoted $100. Versus roughly $60 for 2 appropriate toroidal transformers from Farnell.
Am I correct in my thinking that where physical space allows, multiple transformers is the better approach?
Are there any advantages to a single transformer with multiple secondaries that I am not be aware of?
I'm brewing up a tube preamp and my question regards using a single transformer with both a HV and a heater secondary vs using separate transformers for HV and heaters.
I'm not going to claim I understand all of the issues involved. However from my research on the matter it appears to be that multiple transformers is the better solution.
This appears to be for 3 reasons:
1) Interwinding capacitance
2) Magnetic coupling of noise between the secondaries
3) Cost: I asked our local transformer winder for a quote for a single EI transformer and was quoted $100. Versus roughly $60 for 2 appropriate toroidal transformers from Farnell.
Am I correct in my thinking that where physical space allows, multiple transformers is the better approach?
Are there any advantages to a single transformer with multiple secondaries that I am not be aware of?
I don't think multiple transformers are "better" except to impress the eye. Especially on a preamp which is not a dynamic load.
Transformers are not precision things. You can't even use the output directly! (for audio) You have to rectify and filter and filter, maybe regulate.
It's like lumber for furniture making. You get a piece(s) big enough for the work, saw and plane down to exact dimension. If two 2x8 are cheaper than one 2x16(!), do that.
Transformers are not precision things. You can't even use the output directly! (for audio) You have to rectify and filter and filter, maybe regulate.
It's like lumber for furniture making. You get a piece(s) big enough for the work, saw and plane down to exact dimension. If two 2x8 are cheaper than one 2x16(!), do that.
G'day,
In most cases you won't notice the difference but where one (or more) windings may have a heavily varying load-current (like for class AB or D power amplifiers) and other windings feed (sensitive) circuits with a rather constant load current, you may force supply voltage variations from the first onto the latter. Say for a class AB power amplifier and a pre-amp, I would prefer two separate transformers though the difference may be marginal. You see some highly priced power amps using a separate transformer for each channel, probably for that reason.
For vacuum-tube amplifiers, you are a better judge than I on load-current variations - some seem (to me) to run rather class A.
In commercial production, one bigger transformer is normally cheaper than more smaller when handling is included in the calculation.
In most cases you won't notice the difference but where one (or more) windings may have a heavily varying load-current (like for class AB or D power amplifiers) and other windings feed (sensitive) circuits with a rather constant load current, you may force supply voltage variations from the first onto the latter. Say for a class AB power amplifier and a pre-amp, I would prefer two separate transformers though the difference may be marginal. You see some highly priced power amps using a separate transformer for each channel, probably for that reason.
For vacuum-tube amplifiers, you are a better judge than I on load-current variations - some seem (to me) to run rather class A.
In commercial production, one bigger transformer is normally cheaper than more smaller when handling is included in the calculation.
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Thanks guys,
Since both solutions are going to be much the same now I have a moral dilemma to figure out.
Lower cost and support the big boys off-shore.
Pay double and support the last of the old-timers standing in New Zealand that can do low volume custom transformers.
Wallet vs heart......
Since both solutions are going to be much the same now I have a moral dilemma to figure out.
Lower cost and support the big boys off-shore.
Pay double and support the last of the old-timers standing in New Zealand that can do low volume custom transformers.
Wallet vs heart......
I haven't had the best of experiences with transformers from Farnell. If it's the house brand (multicomp), expect some hum. The medical grade transformers seem to work better, but they're priced accordingly.
What does the locally built transformer have to offer? Gauss band, interwinding screen, varnish? Maybe it's good value after all.
What does the locally built transformer have to offer? Gauss band, interwinding screen, varnish? Maybe it's good value after all.
Thanks for that info. The local guy is looking more attractive.
My next question would be by how much should I over rate current capability of the transformer.
I need 650mA for the heaters. Applying a derating of 0.65 for DC means I would need at least 1A.
Would it be silly to go with a 2A secondary for the sake of temps and regulation?
I am guessing that the extra current handling will not add massively to the cost.
Similar situation for the HV. I need 20mA of regulated DC. I guesstimating 10mA lost in regulation giving 30mA. 0.65 derating would give 46mA.
Would 60mA for the HV secondary be unreasonable?
My next question would be by how much should I over rate current capability of the transformer.
I need 650mA for the heaters. Applying a derating of 0.65 for DC means I would need at least 1A.
Would it be silly to go with a 2A secondary for the sake of temps and regulation?
I am guessing that the extra current handling will not add massively to the cost.
Similar situation for the HV. I need 20mA of regulated DC. I guesstimating 10mA lost in regulation giving 30mA. 0.65 derating would give 46mA.
Would 60mA for the HV secondary be unreasonable?
Sadface, how many valves in total (and valve used 12AX7? 6922?) and what is the HV you need? I presume you need 6.3 V for the heaters, though I have seen a single 12.6 V rail with two valves connected in series. I have an Audion with this config, and they used a 50 VA toroidal with a 200 V 160 mA HV and 12 V 1.5 A LV secondaries. Adequate for two valves - 6922 in this instance. Would a toroidal be okay or do you specifically want an EI? The toroidal is likely to be less expensive.
Indeed, I've used the 70W HV modules from eBay (not short-circuit proof, note!), with adjustable 200--450V output. No idea if they really handle a full 70W, but seem to work reasonably, cheap, light. Add some LM2596's for filament supply, all can be powered from 12V--24V dc.
Its a dual mono aikido. 2x 6SN7 per channel. Regulated DC heaters wired in series to keep the current down.
HV I want 230v so that I have plenty of headroom to regulate down to 265vdc.
I am not opposed to toroidal transformers, I generally prefer them however I am yet to find one with the appropriate secondaries at a good price.
Toroid Transformer H50 50W Series_Toroidal Core_Power Transformer_Analog Metric - DIY Audio Kit Developer
H50-272 would work but by the time I pay currency conversion and shipping its going to be more expensive than the local guy.
I am quite interested in SMPS supplies, however I am very skeptical of ebay electronics after few bad experiences. I would find it hard to trust cheap HV gear.
There is also the part where I would need to get from 230v mains down to 12v-24v meaning either another psu before the SMPS. It looks to me like a lot of extra complexity for the lower cost.
I've been watching this thread with interest
Powerful SMPS for tube ampifier
If they can come up with something mains powered, robust, safe and reliable I will be all over it.