Question on tube load resistant & output trans (need adv

Dear all especially accphoto,

need advice from u all...

Example 2A3's load resistance is 2.5k ohm, so we will use 2.5k ohm output transformer(at primary),

my question is if, we parrallel 2 piece of 2A3 at 1 channel--> do we still use 2.5k output trans or we shall use 1.25k ohm output trans???

besides, i'm also doubt on the type of output trnsformer?


what is the different between SET output trans and push pull output trans?

can i use the push-pull output trans for a SET equipment?

In the case of parallel single-ended outout (PSE), if using two output tubes in parallel, the transformer primary impedance should be halved. Let say one tube single-ended (SE) load for 2A3 is 3Kohm, then two tubes parallel single-ended (PSE) load for 2A3 is 1.5Kohm. Same principle, three tubes parallel (PSE) load for 2A3 is 1Kohm.

Product examples of SE/PSE output transformer for parallelling tubes:
http://www.sowter.co.uk/se.php

There is same principle in paralelling tubes in push-pull (PPP) mode. Let say push-pull output transformer primary impedance for one pair of 2A3 (PP) is 6Kohm. In parallelling two pairs of 2A3 in push-pull (PPP), the output transformer primary impedance should be halved, i.e. 3Kohm. Same principle, parallelling three pairs of 2A3 in push-pull (PPP), the transformer loading is then 2Kohm. Example link here:

http://www.sowter.co.uk/pp.php

You cannot use Push-pull type output transformer in single-end mode.

In single-ended mode, the SE output transformer always has a DC output tube biasing current flowing through. In order not to saturate the magnetic core due to this DC biasing current, a small air-gap is introduced into the core of the SE output transformer. The size of this air-gap is proportional to the DC biasing current. Therefore, the higher the DC biasing current, the larger the gap length.

The gap-length is precisely trimed at factory to maintain the flux density below saturation while at the most linear region of the core. So you must use exact DC biasing current for your SE transformer.

Hence SE output transfromer is specified with a DC current, eg:

2A3 15W/60mA 3Kohm SE
2A3 30W/120mA 1.5Kohm PSE

Example of SE transformer specification can be reference to here:

http://lundahl.se/pdfs/datash/1620_3_7_9202.pdf

Tango catalog

Someone had already answered your question.

thank you very much autumleafcat!!!

If i were DIY a 300B SE as the AN kit 1 in the attachment, is a 12w output trans, max 100mA (balance dc 60mA) sufficient??

besides, instead of 2.5k primary can i use the 3k primary?

Anyone here had experience with SAC silk transformer? any comment?

AN Kit-1 is a single-ended design. Usually the 300B is biased at 70-90mA, loading 2.5K-3.5k. Higher loading gives (slightly) lower power but also lower distortion. 12W trans seems marginal sufficient because single-ended OPT need at least 50% more headroom from rated output power. Single-ended 300B gives 9W output, then the OPT should handle around 14W in order to have better performance.

You mention balance dc 60mA in your OPT. This seem that your OPT is push-pull type? Push-pull OPT will not work in single-end operation.

thanks!

The transformer i'm thinking to use is the silk S-303 from www.sacthailand.com it is a SE output trans.

On behalf of my friend, i have another question, he is keen to know whether a 8-10w SET is able to push his proAc 1sc (86dB)?? he mainly listen to sentimental song & vocal.

Silk OPT:
http://www.sacthailand.com/Transformer_Output.html

10 henry @ unbalance dc 60mA, you are considering the correct trans. That means your transformer best operating precisely at 60mA.

However one last point to remind you is that the only 10 henry primary inductance for this transformer may not give too deep the low frequency response. That make your amplifier a little bit bass shy. Basically the higher the inductance, the deeper the bass the amplifier can dig deep.

Considering the British made OPT, 25H (henry) gives much lower bass.

I also an owner of ProAc 1SC. It can give a true panarama sonic picture even without critical placement. The soundstage of 1SC has all width, depth and height. The musician has pin-pont focusing. It reflect the quality of your amp, source and cable like a mirror honestly.

However, it is not as easy to drive as stated on ProAc web site. It is quite a hungry speaker indeed. I used to drive 1SC by Audion Sterling 14W EL34 ETSE. The amp even cannot handle the dynamic of piano left hand bass note. Maybe this was mainly due to the design limit of Sterling. Using decent 300B SE amp may have much better results.

Finally I drive 1SC by Threshold S/300 150WX2 vintage solid-state amp. This truely unlock the unlimited dynamic capability of 1SC.

As stated in stereophile magazine US web site, the measured efficiency of ProAc 1SC is actually 84dB instead of 86dB stated by ProAc. At 120Hz 1SC appears a 5ohm impedance which is a difficult load for some SE amp. One can say above mid-bass, 1SC can then be considered as easy load.

I love symphonic works (Mahler, etc), so 4 years ago I switch to ProAc D25 which its lower bass is what 1SC could't match. Anyway the 1SC is a legendary pair of speaker and still worth to keep. So I never sell my 1SC, but lend to my best friend who continues the 1SC in healthy listening.

Thank you very much for your info.....

You are welcome.

Last weekend, my friend and i is testing a meixing 300B (4pcs) SE rated 20W on his Proac 1SC... result is dissapointing.... it can't release the real strength of 1sc then we test another amp (Ella, 50W push pull - can use KT 88, 6550 & KT 90), this Ella can really drive the proAc and it had really show the strength of the 1sc.....

Do you have any experiece in hybrid amp?
Since a good output transformer is expensive, i'm looking into the alternative to use mosfet to replace the output transformer, is it can be done?

Hybrid tube power amp is basically totally different from that using OPT, because the voltage drive requirement in the output stage of both design is not same. Pure tube output stage is usually low gain so the driver provides a large voltage swing. However, the MOSFET output stage needs a much lower voltage swing in comparison.

Therefore there should be no direct replacement of OPT from MOSFET.

I had auditioned one of the model of Counterpoint which was a hybrid design (tube driver, MOSFET output, but I forgot the exact model name) in my friend's home. Pairing with a bookshelf Monitor Audio gave astonishing performance, both dynamic and musicallity.

In my opinion, you can try to grab one in 2nd hand market and mod it properly. I think this can satisfy your DIY interest at low risk. Today components are expensive. A DIY amp might cost similar to a 2nd hand branded model. You can also learn circuit design from the model.

i wondering other than output trans, any other buffer circuit can be use to convert the ouput of the tube to the speaker?

How does OTL(output transformerless) amp functioning?

what kind of circuit the OTL use to convert the high impedance load to a low impedance load(speaker)?

Do you think the circuit attached can be work out?

i have several 2C51, 5687 & 12A6 tube.... tinking of make use of them to DIY a OTL...but i'm not so understand the way the OTL work, are you familiar with OTL circuit?

OTL can give transparent sound but the main drawback is lacking true bass, due to its inherent characteristics of which, the lower the load impedance, the lower the OTL power output. You know for normal amp, especially solid-state amp, halving load impedance obtains double power output. So you see the transistor amp spec as 100W @ 8ohm; 200W @ 4ohm. But for OTL you will see spec in reverse like this: 100W @ 8ohm; 50W @ 4ohm. However I know just little on OTL.

I am using a DIY 300B driving a pair of JM Lab 926 modified with premium parts of course. the amp can drive the speaker with flying colors. I think whether a SE amp has enough power to drive a pair of speaker is really depends on the design and parts of the amp. However, my case is a abnormal one, so do not count on it.

Agree to accphoto, decent 300B amp can make miracle happen. Meanwhile, the high 90dB sensitivity and constant 8ohm impedance of your JM Lab also benefits to exploit the mystery watts of 300B.

By theory, for every 3dB rise in speaker sensitivity, the speaker needs just half the amplifeir power to create same SPL (Sound Pressure Level). That is for say, if 7W 300B driving 90dB sensitivity speaker can satisfy you, then 84dB sensitivity of ProAc 1SC would need at least 28W power for same SPL. Sensitivity acts a role as well.

(power X 2 for 3dB difference, power X 4 for 6dB difference)

Wow, thank you very much for the info.

Just bought a 2nd hand Monitor audio 9 gold speaker and modified with OIMP. My previous speaker MA R100's bass is too soft as its low pass filter is using single inductuctor only.

dear accphoto,

Do u think changing the iron core inductor to foil inductor in crossoverr will give more improvement compare to changing the cap?

any of you had experince on the obligato film cap?

I have a doubt, my orginal bipolar cap for low pass filter is written as 6.8uf but it read/measured as 7.2uF, shall i use 6.8 of 7.2?

currently i found my HF is very good, bass also plenty but mid seems like a bit lacking, if i use 6.8uf do you think the mid will be better?

Number of components using in speaker crossover depends on designer mind. Some may prefer lower order (simple network) which gives more natural sound. Complex network usually gives better overall frequency response which results in balance and refine sound.

There is more number of turns of copper wire in air-core inductor than that in iron-iron inductor of same inductance, since the iron-core can increase the inductance with fewer number of turns of copper wire.

Therefore, the internal resistance of air-core inductor is higher than that of iron-core one. Replacing iron-core inductor by air-core one, the difference in internal resistance would result in tonal inbalance.

If iron-core inductor being replaced is at woofer, increase of internal resistance of air-core inductor would reduces bass energy. I experienced this in modifying my old Rogers in childhood.

However, foil-type air-core inductor has lower internal resistance than copper-wire type one of same inductance. This releases the internal resistance issue. The principle of replacing inductor is that, not only match the indutance, but also match the internal resistance. Always maintain the same parameter set of original components. Certainly sometimes mis-match may give surpprising result, you can try.

Generally air-core gives very wide stage and pure sound than iron-core because the iron-core material produce hysteresis distortion. But distortion in iron-core may induce so-called "taste" of loudspeaker. Famous case is LS3/5A which has many iron-cores in the cross-over.

Capacitor is usually specified as +/-10% tolerance, the error in 7.2uF is within the tolerance of 6.8uF. For serious speaker builder, the error is being calculated into the design formula. So putting a precise 6.8uF might worsen the case or might not. But match to the origin capacitor value is more safe. You won't have risk on shifting the crossover point.

Capacitor dielectric (material of made) does has great change in sound. This is also what I experienced in modifying Alon II speaker many years ago. At that time I used Solen and Wima only because of cheap. The result is pin-point focus and plenty of texture. Solen is not bright so the high frequency is not being emphasized and beautifully round. But a bit cool in overall, not too warm. Wima might be a bit bright but it gives real timbre that make live recording very enjoying.

Hovland Music-cap may make mid-band more obvious with smooth high, as reported by my friend. Remind you that there is dedicated Hovland Music-Cap for loudspeaker crossover. Give it a try ......

Thank you. U had solve part of my doubt.

For the inductor, is the internal resistor only affect the bass energy, would it affect the frequency cut off point?? (this is what i'm worry). Does it mean that using an inductor with less internal resistor, the sound will be faster will have more energy??

I had some experience with solen cap, it is quite smooth & warm but blur, poor imaging & sound stage.

For the tweeter (HPF) i will recommended to use Vcap OIMP if affortable. For cheap replacement Mcap MKP is not bad.

For mid bass, i'm still searching for nice & cheap cap, Mcap Zn is quite warm but not so cheap. Hovland Musicap not yet try as also not cheap at my place and heard that it not easy to match...

The internal resistance of inductor is of the order of less than one ohm. Hence this should not affect crossover frequency point seriously. In my experience, putting inductor of higher internal resistance in woofer side makes treble sound louder in comparison to bass. If you like faster sound, air-core (both copper wire or foil type) is good choice. Just take care of internal resistance issue. You can try. Maybe in your case the problem of internal resistance is less than mine.

I used Solen in my Alon II modification more than ten years ago. At that time Solen was made in France but now in Canada, ... different !

Anyway, nice talk with you ...

Hi, do u have any idea the purity of the copper wire in the inductor use by monitor audio in 1990s? is it 99.9% or 99.99% cu?

I have a problem here as my multimeter is not so good it can't really measure accurately the resistance which < 1 ohm.

I'm planning to use Jantzen 99.99% cu air core inductor, if my original iron core is 99% or 99.9% pure then i guess the internal resistance might be similar even i use a air core coil.

If possible i try to avoid using the foil air core as the price is double of the aircore wire coil...

To measure resistance near 1 ohm accurately needs professional four-wire (Kelvin probing) multimeter in laborary, which is expensive.

I don't know what quality of the inductor of Monitor Audio speaker.

You may try putting a very small value resistance in series to tweeter to balance the sound level difference after you change the inductors.

ok, thanks for sharing.

By the way, what is your opinion on using combination of 2 or more cap to reach a desire value for speaker cross over cap?

e.g 3.3+4.7uf or 6.8uf+0.2u to become 7uf compare to 1 pcs of 7uF.

Often i prefer to use a single cap compare to multiple cap.

In my case now my measure value is 7.2uf, it is really difficult to get a 7.2uf single cap.  So i'm wondering i shall just use a 6.8uf or use 6.8+0.47uf??

lastnight i replace my 7.2 bipolar cap(at woofer) with (6u+1u) mkt cap, mid & bass become very transparent, clear & better imaging but need to sacrify some warmness & bass quantity....

Any suggestion of cap which can give good imaging, resolution and bass and still having warm sound?

It is possible to parallelling multiple cap to get a desire value. But certainly you must have a meter to verify your combination.

I think you can try 6.8uF first. If there is no drawback on sound quality, that means your speaker is not sensitive to tolerance.

In fact I have tried some brands of cap which is not famous but still foriegn made, including GE, philips, RIFA at a low cost from internet shopping (RS, farnell ... ). The result is quite good. Especially some WIMA MKS which is much cheaper than normal high-end MKP10 series. I found many famous speaker using cheap cap have very natural balance sound. So I think famous branding cap has too much personality that suit for particular taste and charge prenium.

hmm... will try a 6.8uf as you suggest, if not ok then only parrallel in some cap....
i think the requirement for cap use in woofer is not as critical as in tweeter, thus shld be able to try cheaper cap to get decent sound...

Even cap in woofer would affect the mid-band quality. In my old case of Alon II, which was a 3-way speaker. I changed the non-polar e-cap in woofer to cheap solen (still french made). The focusing in mid-band also improved a lot. The woofer in Alon II is of 12-inch diameter. It supposed responsibile for under 400Hz. But the result told me that the change in woofer also improved mid-band quality. Just sharing ....

Hmm, i had just get my brother to buy me the Jantzen foil inductor in order to replace the iron core inductor in my MA 9. Hopefully the foil inductor have low impedance enough to further boost up/ equall my current bass.

Dear ALC, acphoto,

If u you 2 choice for inductor below with similar price, what will u choose for the series inductor in woofer and why?

1) Foil inductor, air core, 16AWG, DCR = 0.34R
2) air core wire inductor, 14 AWG, DCR =0.25R

I will describe the differrence of two coils before making a choice.

For coil 2, it has thicker wire (14AWG) and lower resistance. This coil is at woofer side. It mainly flowing low frequency current. However, if your speaker is a 2-way system, it will passing current up to mid-band and also beyond cross-over point slightly (>3kHz). Skin-effect in wire-type geometry will increase resistance at high frequency, though in theory it is very little, perhaps in the order of 0.001 ohm. So many branded loudspeaker would use wire-type coil for economic reasons.

In physics high frequency currents tend to flowing on surface (skin) of conductor due to complex dielectric and permeability related problem. Skin depth reduces and thus increasing resistance as frequency increases. Skin effect is talking about resistance of wire increase with frequency but only significant above 1MHz. Therefore, the effect in audio frequency is tiny. But some people says the resistance variation due to skin effect is considerable in the sense of magnitude difference across full audio spectrum (20-20kHz). For example, wire resistance is 0.0001 ohm (say) at 20Hz, the wire resistance becomes (say) 0.001ohm at 20kHz. However there is still nobody conducting an meaningful experiment on this issue. Theory is always correct. But in practical audio matters, it is no idea how skin effect plays an role.

For coil 1, the advantage of foil type geometry is better current transmission at high frequency, due to larger surface area of foil geometry than round wire of same cross-sectional area (AWG gauge). One would expect foil type has lower resistance rise at high frequency. The coil performs more linear across full audio band.

The comparison is obvious because two coil has different gauge. The DCR of the two coils are significantly different. I think DCR of coil 2 gives more advantage regardless of skin effect issue. For coil 1, the advantage in skin effect may be upset by loss in bass energy.

Coil 1 may sound more beautiful because of foil geometry and thus less variations in resistance across audio band. But maybe I have not enough concrete proof to support my hypothesis. Anyway you don't need to take my words so serious. Just be an experimenter and try.

Thanks !

Thanks, your analysis make sense....

My system is 2way speaker.

So this round is try & error =  gambling. I hope my newly purchase 16AWG foil inductor will have equal or lower DCR than my original iron core inductor, then sound quality & bass will be better than current.... then i'm already satisfy...

Good Luck and share with us your finding.

dear ALC,

Another doubt need your advices.

Do u ever try to DIY a preamp/tube buffer using power tube?

can we actually build preamp use power tube like 2A3, 12A6, 6550 and so on? what will be the impact?

I know Manley (VTL, I'm not sure) use 300B as preamp. But I never heard the sound. I think only DHT like 2A3, 300B have good linearity to build a preamp. Other power tetrode like 6550, etc is poor linearity.

Hi, a question.


The LF hum in tube preamp is cause by the unsufficient capacitance in power supply filtering right? This LF hum will increasewhen voulme level increase.

This is one of the possible reasons. There also ground loop issue that hum current flowing along the ground rail of small signal path such as volume control and selector, thus amplifying the hum. Seperate small signal ground and power ground to eliminate cross coupled ...

U r right, by just changing the connection point of 1 ground wire, i had reduce a lot on the hum.....