The change of reflected impedance in the primary of a transformer - Page 1
Forum section: electronics / beginners

author: nenea dani
author_rank: Regular Contributor
timestamp: October 11, 2022, 09:30:23 pm
content: Hello colleagues,I have a new toroidal output transformer manufactured for an amplifier with an electronic tube in single ended configuration. The primary has 7 KOhm and in the secondary I can connect speakers with 4, 6 or 8 Ohm. I would like to use this transformer so that the primary has around 3.5 KOhm. I can connect the 8 Ohm output to one 6 Ohm speaker in parallel with a 12 Ohm resistor, obviously with reduced power. What could change in the primary circuit in this configuration? The transformer was designed for 30W, but I want to use it only at 5-6W.

author: Zero999
author_rank: Super Contributor
timestamp: October 12, 2022, 05:38:51 pm
content: Use a 4 Ohm speaker and the input impedance will be halved.

author: nenea dani
author_rank: Regular Contributor
timestamp: October 12, 2022, 06:18:41 pm
content: I understand, but I have a pair of speakers on 6 Ohm with a very good frequency response( Dynavoice Definition DM-6) and I want to make a stereo amplifier. I have three amplifiers under development, one with ECL86, another with EL84 and another with EL34. That's why I want to be able to change the impedance of the transformer. I want to find out which one sounds better.

author: Zero999
author_rank: Super Contributor
timestamp: October 13, 2022, 05:35:02 pm
content: Connect them in parallel. If you need stereo, buy another pair. Otherwise use a different transformer.

author: eugene
author_rank: Frequent Contributor
timestamp: October 21, 2022, 06:30:22 pm
content: Have you tried connecting the 8 ohm speaker to the 8 ohm tap for a primary impedance of 7k? It will almost certainly work and might be completely adequate for your needs.I suppose you could parallel the 8 ohm speaker with a 8 ohm power resistor...I'm curious about a toroidal transformer being used in a single ended application, or even push-pull for that matter. They do not tolerate any DC current. Is it ac coupled in this case (a.k.a parafeed)?

author: nenea dani
author_rank: Regular Contributor
timestamp: April 23, 2023, 09:14:20 pm
content: I have done many tests in the last months with Toroidy transformers and they behave very well with DC currents of 70-80mA. It does not appear saturated, not even at low frequencies. I connected a fixed resistance of 6 Ohm in the secondary of a 3.5k/6 Ohm torus and fed the primary from a sinusoidal generator with very low output impedance. Between 20Hz and 20kHz I measured a very small impedance variation around 3.5kOhm.

author: radiolistener
author_rank: Super Contributor
timestamp: April 23, 2023, 09:28:57 pm
content: For transformer impedance ratio is a square of voltage/turns ratio.For example 1:4 impedance ratio means 1:sqrt(4)=1:2 voltage/turns ratio.If you want to load transformer secondary coil with 8 Ω and have 3500 Ω at primary coil, then you're needs impedance ratio 437:1 (3500/8). It means that turns ratio should be 21:1. So, the primary coil should have 21 times more winding turns than secondary coil. Such transformer will have 437:1 impedance ratio, so when you apply 8 Ω load at the secondary coil, the primary coil impedance will be 8*437=3496 Ω

author: MathWizard
author_rank: Super Contributor
timestamp: April 25, 2023, 02:55:21 pm
content: Quote from: eugene on October 21, 2022, 06:30:22 pmDo you mean pull-pull amps and speakers don't like about DC, or that toroidal transformers don't like DC ? If it's the trans, why not ?

author: Xena E
author_rank: Frequent Contributor
timestamp: April 25, 2023, 04:02:00 pm
content: Mismatch of a tube/valve output transformer isn't such a problem as some think, it really first and foremost affects power transfer.As for the standing current, the OP say they have transformers that are greatly over spec for their needs, so there shouldn't be such a problem with standing current in a single ended design in that case. Of course a toroidal design can not be retrospectively "gapped".

author: The Electrician
author_rank: Frequent Contributor
timestamp: April 25, 2023, 07:09:59 pm
content: Quote from: Xena E on April 25, 2023, 04:02:00 pmWhat effect does it have on power transfer?

author: TimFox
author_rank: Super Contributor
timestamp: April 25, 2023, 07:19:16 pm
content: Quote from: MathWizard on April 25, 2023, 02:55:21 pmIn a push-pull amplifier, the quiescent net DC flux in the core is zero, since the two equal bias currents flow in opposite directions with respect to the winding.In a single-ended amplifier, the quiescent current (which for the required class-A amplifier is large) makes a large net flux in the core.Therefore, transformers intended for single-ended use must have a gap in the core, just as the core gap in an inductor designed to carry a large DC current.Toroidal transformers usually do not have gaps, and therefore must have relatively small net DC current in the primary winding.If the net DC flux is high enough, the transformer core will saturate on positive peaks (suitably defined) and cause distortion.

author: radiolistener
author_rank: Super Contributor
timestamp: April 25, 2023, 08:32:39 pm
content: Quote from: The Electrician on April 25, 2023, 07:09:59 pmunable to push enough power into a load due to impedance mismatch

author: The Electrician
author_rank: Frequent Contributor
timestamp: April 25, 2023, 09:39:26 pm
content: Quote from: radiolistener on April 25, 2023, 08:32:39 pmDo you agree with Xena E that the first and foremost problem due to "mismatch of a tube/valve output transformer" is the inability to push enough power into a load?

author: TimFox
author_rank: Super Contributor
timestamp: April 25, 2023, 11:04:18 pm
content: In the specific case of transformer-coupled vacuum-tube power amplifiers, the load impedance reflected back through the impedance ratio gives you a "load line" on the graph of plate characteristic curves.This is a very important point, and I refer you to the textbooks to understand the details.For a single-ended tube, the total current cannot go negative, and the maximum positive excursion normally is limited by the plate current vs. plate voltage curve corresponding to 0 V grid-cathode voltage.The slope of the load line allows you to obtain the maximum power limited by your plate supply voltage and these curves.

author: Xena E
author_rank: Frequent Contributor
timestamp: April 25, 2023, 11:35:57 pm
content: To add... what many think, is that the load impeedance, (speaker/transformer combination), defines the work that the tube/valve has to do, however as Tim points out, what power is available to the speaker it controlled by tube type, bias, HT "B" voltage etc...How good the matching is only defines the transfer of useful power to be available to the speaker, within reasonable limits of course: what the OP is suggesting seems "reasonable".The main caveat with a single ended tube power amplifier is that you should not load it to lightly, or leave the output open circuit as there is a risk of high transient voltages destroying the primary of the transformer.

author: The Electrician
author_rank: Frequent Contributor
timestamp: April 26, 2023, 01:00:02 am
content: Quote from: Xena E on April 25, 2023, 11:35:57 pmThis is not true.DIY builders of vacuum tube audio power amplifiers know that distortion and frequency response are affected by matching between the output stage and the output transformer. It's quite common for those two parameters to be optimized by some amount of mismatch, and some tradeoff between power transfer and sonic performance is usually considered to be worthwhile.

author: TimFox
author_rank: Super Contributor
timestamp: April 26, 2023, 03:41:49 am
content: Quote from: The Electrician on April 26, 2023, 01:00:02 amI think the linkage of matching to power transfer is a common misunderstanding of the Maximum Power Transfer theorem.That theorem is relevant to truly linear circuits comprising voltage or current sources and impedances.However, for a tube amplifier and DC power supply, the maximum power output does not correspond to a load impedance equal to the device output impedance.Specifically, look at the RCA datasheet for the 2A3 power triode:https://frank.pocnet.net/sheets/021/2/2A3.pdfOn page 52, you see the recommended 2500 ohm load line added to the family of plate curves for maximum power output, corresponding to the "Single Tube Class A1 Amplifier" tabulation on page 51.The actual output impedance of the device with those operating conditions is the "plate resistance", which equals 800 ohms.If you compare this to a 6V6 beam-power tube, you will get a similar optimal load impedance for a device plate resistance that is much higher than the load resistance.https://frank.pocnet.net/sheets/127/6/6V6GTA.pdf5500 ohms load with 50,000 ohms plate resistance.

author: Xena E
author_rank: Frequent Contributor
timestamp: April 26, 2023, 06:12:29 am
content: Quote from: The Electrician on April 26, 2023, 01:00:02 amPlease note I said within reasonable limits. If the OP has six ohm speakers and connects them to to a four ohm output for instance, it would not make any practical difference. Distortion performance degradation is caused by too high load resistance causing lack of damping: too low resistance obviously causing loading effects.His questions were quite specific.Offering reduction to absurdity answers just to showboat isn't helpful.

author: The Electrician
author_rank: Frequent Contributor
timestamp: April 26, 2023, 03:06:06 pm
content: Quote from: Xena E on April 26, 2023, 06:12:29 amThat is what you have done when you allow power transfer as the only effect of impedance mismatch, reasonable limits or not. If you want to be helpful you would discuss all the possible negative consequences of impedance mismatch, rather than reduce it to only power transfer.It is absurd to think the OP wouldn't care about other negative consequences of mismatch, especially people who are considering a single ended vacuum tube power amplifier. People who bother with such an amplifier have done so because they care about the quality of sonic performance obtainable.

author: Xena E
author_rank: Frequent Contributor
timestamp: April 26, 2023, 04:25:22 pm
content: Quote from: The Electrician on April 26, 2023, 03:06:06 pmNo. The reasonable limits are very wide, discounting power transfer.Getting wrapped up in multiple decimal place calculations is truly absurd when the impeedance reflected through the output transformer from the speaker is varying widely based on frequency, resonances, acoustics in the environment it is operated in even,It's good to get things close but a lot of effort went into the development of negative feedback for a reason. The biggest source of nonlinearity distortion and poor frequency response is created by the loudspeakers.Golden ears and audiophool opinions are of little use to an experimenter with a limited budget to spend on components. It only serves to confuse. A good transformer for a given output duty in a tube amplifier requires very careful design to get the very best performance, that's something that takes time and skill, or vast amounts of cash.When I hear terms bandied about like 'sonic performance' I know the type that utters such tripe.If you want HiFi don't build with tubes. And don't build single ended output stages. If you want something that sounds like a tube amplifier and has rich second harmonic distortion because you like it go right ahead.Being a self righteous twat and itching to make a put down to make yourself look big, is frankly pitiful....Best regards.

author: The Electrician
author_rank: Frequent Contributor
timestamp: April 26, 2023, 06:06:10 pm
content: My responses to you have only dealt with what you say in your posts.Both your responses to me have indulged in name calling and personal attack. Your arguments are not strengthened by this behavior.