Dynaco ST-70 Original vs. VTA Bench & Listening Test Comparisons - Stereo 70 Tube Amp

[@DynakitParts]

I'm not surprised by these results.....What surprises me is the fact that it has taken more than (20) + years before someone actually tested the VTA ST-70 upgrade circuit & compared this to the original David Hafler ST-70 design. Nice job Ben! Kevin D. from Dynakit

[@julesl6910]

Great channel, found you thanks to Audiophiliac's video that dropped today. Just ordered some of these amps!

[@dangrass]

Really excellent piece. Thanks for doing this. Makes me appreciate my original Stereo 70 all the more.

[@williamsmith306]

The original circuit was often derided and considered to be inferior to any newer modified driver boards.This as some have speculated was purposely promoted by the makers of those driver boards. Bench testing actually shows that the original design is quite excellent up to full rated power. Yes different driver boards will have different sound, but one would have to try many variations in order to see if they are actually preferable, and as you yourself discovered it can vary from song to song. I consider the OEM circuit as designed to be a real ST-70, the others are just similar to the ST-70. Great video by the way.

[@newardthelman6871]

Finally, blind listening tests, rather than the usual audiophile fairy dust BS. Super interesting bench tests. This is probably the very best KZbin audio review I've ever seen. Thank you.

[@Darrylizer1]

My first and only kit thus far is the VTA ST-70 that I've tried with both EL-34s and KT-88s. I prefer the EL-34s, but I was surprised that the VTA had worse distortion characteristics at a far lower wattage than the original design. That threw me a bit, but I love the VTA, it's a fantastic sounding amp and is currently my daily driver. I also have my dad's Dynaco Mark III monoblocks that I restored to original specifications, only replacing the selenium rectifiers with diodes. I too discovered your channel because of Steve Guttenberg The Audiophiliac's video.

[@NosEL34]

Awesome video... thoroughly enjoyed it & thanks for putting in the work. I appreciate the sound & presentation of the original circuit but when it comes to all the "mod" units available i enjoy the Van Alstine Ultimate 70 circuit. He still sells the PCB for $50. The topology is basically the same as the original but precision parts (not boutique) are recommended in certain areas. A good rectifier is needed but using different output tubes dont seem to alter the sound much. Frank wont go into detail about the improvements he made but his efforts focused on the time constant ratios,feedback,bandwidth, & power supply. Each power tube having its own PS cap. I dk how it all ties together but I do find it to be a proper sounding amplifier with low distortion/noise. I would love to see it tested.

[@tubes4hifi]

Thanks for the review Ben! It's great to see both the bench and listening thoughts. If you'd like to try the 6SN7 board, let me know.

[@NovaluxStereophonic]

@@tubes4hifi unfortunately I think the power transformers on these units are too small? PA060. Perhaps I could build a new kit from scratch instead… Any comments on the distortion analysis?

[@johnparichuk8367]

I built the VTA ST-120 seven years ago. It runs 6550 output tubes. I made a few changes. I went to dual rectifier tubes long before the modification was available on the website. I was having problems with the center 12AU7 tube shorting cathode to filament. I replaced the driver board with the 6SN7 version. to my surprise, the 6SN7 version sounds better to my ears. Aside from that, the amplifier has performed flawlessly and sounds great. I am also running the VTA SP14 line preamp and VTA PH16 phono preamp. Both pieces are excellent.

[@HackingSMA]

Terrific video! I really appreciate the detail and thoroughness. One thing that might be worth pointing out is that 35W is only 4.6dB louder than 12W, which might be less than intuitive for some. In any case, I'm curious, do you have any vague quantitative sense of how hard you were pushing the amps in the listening tests? Thanks again for the excellent video.

[@mcgjohn22]

nice video and review on comparing the two amps. Have owned quite a few of the original ST70s over the years. Interesting comment on the difference in feedback between the two versions. This could be accounting for the sonic differences heard between the two amps. As a follow up might be interesting to measure open loop gain, then measure gain again with each amp's feedback employed to see how much feedback was used in each design. Was surprised to see those output transformers could go that low in freq.

[@NovaluxStereophonic]

I am seriously considering doing just this!

[@mbilden]

Always a good day when Ben releases a new video! Thanks.

[@scottspencer4018]

Great video Ben! I've used both versions for many years. While both are indeed nice sounding amps, the VTA version has better dynamics, better clarity, better separation, better bass and treble extension,, and is closer to a high dollar modern tube amp. Honestly, IME just about every sonic category you can name, with the exception of smoother favors the VTA. The VTA version also offers a lot of tube rolling options....I run only one side of each as single channel monoblocks, with a 12BH7 in the center, and Tung Sol 7581A outputs in triode, and a solid state rectifier for about 17 wpc.

[@kelvinjinxd]

Great review. From the schematics, I kind of figured out the original st70 should be better. It is good to see you did actual measurements on them. Good job.

[@tashpaug]

Ben I have had both and a third variant that uses the VTA board with 6SN7 tubes. I prefer the 6SN7 tube variant using KT 77 power tubes. Very good precise placement and great depth through the whole frequency range. Great video!

[@jfritzy4358]

My VTA board with Sylvania NOS 6SN7 and KT77 tubes keeps brining me back from a beast SS amp. KT77 tubes more dynamic than EL34. Sylvania 6SN7 tubes another step up in sound quality.

[@johnminassian4887]

Very interesting. I have never heard Dave Hafler's original design. But I built the VTA ST 70, and I really like it. My tubes amps are not used during the summer. As you know the ST 70 dissipates quite a bit of heat. I shift over to my diyaudio First Watt clones during the summer. They run Class A, but the heat is nothing like what comes off the ST 70. There was a time when I would only listen to tube amplifiers, but Nelson Pass's designs have caused me to reevaluate solid state electronics.

[@satanbane]

Very interesting, thank you for this. I've had an ST-70 for years and always thought it sounded good (I've used it for bass and guitar as well as hi-fi), but frankly, I never knew its specs were so good numerically. One comment, for listening tests, it'd be interesting if you could involve some women as well; they seem to have very different hearing from men, as a general rule. Whenever I'm working with audio stuff, and particularly with my own playing and recording, I value the comments and observations of women: they frequently notice things and point out things which seem obvious once my attention is drawn to them, but which I hadn't been paying attention to before. Different perspectives.

[@PGHJR002]

Very informative video. I never owned the original ST70, but, my first tube amp was from Panor, named the ST 70ll, memories of which I still use as fond references when doing vintage equipment comparisons to this day.

[@chrisblock6697]

Great job. Nice setup behind you. Would love to see a video just going thru your test equipment. Those mounted Mac's also look nice!

[@NovaluxStereophonic]

kzbin.info/www/bejne/sIXZd3Sjl89_i9Esi=6yuFkJSUqTJWyF3P

[@chrisblock6697]

@@NovaluxStereophonic Perfect! Thanks for the quick reply!!!

[@jeffbrooke4892]

Very cool. I've did both amps, but to no where the level of expertise that you have. And in comparison I felt the VTA sounded more like modern tube amps with a larger soundstage and the ST 70 sounded a bit more collapsed and intimate. Both good, but in their own way. And I wrote this down before I got to the listening portion of your video. Great program, thanks!

[@quiksr20]

Great video, I always keep my eyes open for a clean Stereo 70 to to get my feet wet with Tubes. I hear its one of the best for the money, well supported and affordable to start with.

[@NovaluxStereophonic]

@@quiksr20 All true… a great way to get started with Vacuum tube amps.

[@stevenjackson8226]

Fun. Thanks for this. Great stuff. I've had lots of experience with Dynaco gear. Lots. And I've been tracking the conversation around Dynaco gear for a long time. I still have my father's ST-70, PAS-2, and FM-1/FM-3. Quick question. Is the stock 70 in the test using it's selenium rectifier? I didn't see it in the tour of the undersides of the amplifier. The feedback debate... good? bad? the debate continues??

[@NovaluxStereophonic]

@@stevenjackson8226 bias rectifier swapped to silicon.

[@ttownscott]

This is very interesting to watch. I'm actually doing a Original and VTA upgrade of two heavily modded amps I picked up. I don't have the test equipment but looking forward to doing to listening comparison. I did order new larger power transformers and will put the rectifier diodes in on both. My vta will have the 6sn7's and the refreshed original board from soundvalves that uses the 6GH8. I wonder if the driver tubes is what may drive listener preference over the power tubes.

[@Wil_Bloodworth]

Very nice, and different, video Ben. Thank you!

[@sichumhoo6569]

thank you for the presentation. i have not seen such an in depth comparison. truely a learning experience for me. i have an all original restored and a vta with 6sn7 and kt77, would loved to see that on the test bench.

[@lbbork]

Great test. Thanks for taking the time to share.

[@tombranch6983]

Excellent review. I was hoping for a commentary concerning the VTA's higher measured distortion and how it might correlate to what the listening panel heard. Maybe the sound level was not pushing the VTA amplifier, so the higher amount of distortion was not revealed.

[@AudiophileStooge]

Fantastic comparison! Very well done.

[@briansilcox5720]

I am a big fan of the VTA products. Built the VTA 120, updated with dual rectifiers and bias meters. The dual rectifier mod greatly improved reliability.

[@peterlundskow4061]

My first step into the Hi Fi world was to buy a VTA update version about 4 years ago. I didn't think at all at the time there would be a difference in sound or performance. What I was thinking about was to buy a highly reguarded vintage tube amplifier that was checked over & restored by someone with a good reputation for the best reliability & longevity. I just discovered your channel & this video, very interesting & informative! I was surprised that a amplifier with a new 'updated' preamplifier curcuit would have more distortion than the original. It makes me wonder a bit if I should have bought a restored only orginal?

[@nick_yt23]

Does the iPhone internal DAC has high resolution capabilities?

[@ToobTime]

Very interesting, I have a gut rebuilt ST-70 and I used quality and precision parts as well. But I don't think the VTA is really an ST-70, it is a completely different design and model It sounds to me that the VTA is enhancing (amplifying?) higher frequency but the tests show a similar frequency response. I have heard the 'missing" comment on my ST-70. What components would make this change in the sound of music? I guess this is the classical question for tube amp design. It almost sounds to me that there is an element of amp design that behaves differently based on the content of the music. Would an equalizer help to tune songs? (That would be a hassle!) What are your thoughts?

[@NovaluxStereophonic]

@@ToobTime the circuits vary in driver tubes, constant current source use, bias implementation, and negative feedback. I agree, very different amplifiers, the only thing common between them is the power and output transformers.

[@NuttySwiss]

I have an upgraded vta with kt88’s and 6sn7’s, I’m curious how that compares. 😊

[@rickg8015]

Great vid.. I wish you had the Uprated Power Transformers for both amps. It runs cooler and makes a big difference, at least subjectively/sonically.. I have a not so common aftermarket Dynamutt Octal 6SL7input>6SN7PIBoard on my ST70.. And the the B+ supply caps (2 of them) are polypropyene, the other two are dual-section lytics (top mount) with small polyprop bypasses.. The Feedback R is user replaceable, with value recomendations for the Dynamutt board (basically a Mullard circuit with discrete solid state CCS on the PI 6SN7 cathodes - for forced balance) I chose the low FB setting since my speakers are not too hard to drive.. I’ve listened to a really souped up VTA and it projects a holographic image while retaining that El34 mellow tone.. Try replacing those stock metal films with tightly matched Allen-Bradley CC for the grids and plates.. Then do the same for the phase inverters if you still find the Joplin track harsh..

[@ceylonmooney]

very nice. oh my. 12WPC. u know, im not too surprised about the real life performance of the bob latino amp. i came across this after reading a "teaching VTA M-125 to sing," on audiokarma where one of the members took a good, clear look at VTAs grandson of the dynaco mkIII. too bad you couldnt compare the original to the the van alstine ultravalve--thats the top dog among the many descendants of the dynaco ST-70. its be nice to see a direct comparison.

[@meraydin1]

Hi Ben, would you describe how you set up the oscilloscope for left/right residual distortion?

[@NovaluxStereophonic]

My distortion analyzer has an output on that back that can be plugged into an oscilloscope. When measuring distortion it sends out a waveform that shows what’s left after the fundamental is removed. Most distortion analyzers have this capability. It is sometimes called a function output. Hope this helps.

[@meraydin1]

@@NovaluxStereophonic Thanks a lot, I noticed that afterwards :)

[@dloorkour1256]

Nice detailed review, thank you. I would have liked to see 2 things to even the playing field a little more: bias both at 50mA per tube (that 40mA spec on the VTA is just a preference by the seller of that amp for fewer runaway tube issues), and have similar added capacitance on the stock board version compared to the VTA. On a side note, I've found some of the elevated 2nd harmonic distortion devices sound exciting initially, but as I live with them, I become a little tired of their 'character'.

[@NovaluxStereophonic]

@@dloorkour1256 Others have suggested increasing the bias, I wonder if this might help the available power before 1% distortion.

[@poormanselectronicsbench2021]

Nelson Pass, after tinkering with mostly transistor amplifier designs for over 50 years, has come to the thought that, a listener's perception of depth and width of soundstage can be altered by the amount of second harmonic level distortion added to a reproduction of a music selection. He has some great interviews on Steve Guttenberg's youtube channel

[@defec1]

Please change the feedback resistor on the vta70 board to something in the 5-5.7k range and retest it if possible. it would be very interesting to see if it fixes a lot of the issues that it had over the stock circuit or not. thanks.

[@supergwizzo]

No need to even have to listen to know the vta would be clearly better all around 😮😊 good video

[@DynakitParts]

What video did you watch?

[@drde4010]

Nice review, but there doesn’t appear to be any mention of speakers used.

[@NovaluxStereophonic]

@@drde4010 I mention Clements but I did not state the model number.

[@drde4010]

@@NovaluxStereophonic Thanks for sharing and please don't get me wrong, your results are exactly what mine have been, but there's an issue here. You can see the countless Dynaco videos on my channel for yourself. The issue that really bothers me is why end users are so secretive about the speakers they use as if this isn't an important issue. This has a HUGE influence on the results and I get this same response with Amir at Audio Science Review who doesn't share the model of speakers used or even mention the speakers used, why? You're a technician, so you would know even better than I do about why this matters. Different impedance loads, different sensitivity, different brightness, etc. I've never heard Clements speakers, but I wouldn't doubt they are not as bright or revealing as Klipsch Heritage speakers. As a result, matching either of these amps matters greatly. If Klipsch speakers are bright, then the more laid back sound of the original ST70 circuit makes more sense in my opinion, which is why I sold my VTA in favor of the original circuit design. If I combine the VTA with the Heritage line, then the combination is too bright. So now your audience is under the impression the original circuit sounds laid back and it does, but not necessarily so with speakers like the Heritage. I just don't understand why speakers play some a minimal role as a factor when judging amp compatibility and sound quality. It's a very important factor.

[@NovaluxStereophonic]

@@drde4010 The bottom line is this was for fun. I emphasized that everyone’s ears are different. Obviously the people involved in the test disagreed on preference. I try to stick to objective tests on my channel and rarely comment on “sound” this was an experiment that was fun to do with some friend. Take everything I said with a grain of salt. The test results tell the better story.

[@drde4010]

@@NovaluxStereophonic I understand and thanks for sharing.

[@carlitomelon4610]

​@@NovaluxStereophonic Why? These probably couldn't drive Maggies, right? Should I state the model too? 😉

[@RobertKohut]

Great video>

[@poormanselectronicsbench2021]

Interesting comparison, but my choice would be the original, just because, well, its original, less work and expense to get to the final product. A great orchestra / symphony test album, IMO, is the Telarc recording of Fredric Fennell conducting the Cleveland Symphonic winds( with a GOOOD orchestra bass drum) for a selection of Holst suites as well as Bach's Fantasia in G minor and Handel's Music for the Royal fireworks - great production, as it will show weakness in the top and bottom ranges of an amp & speaker selection.

[@neps4th]

As far as VTA shrillness on some voice; try some different type of coupling caps between the tube stages.

[@kinura26]

i think the VTA board recomennds setting the output tube bias cooler than the stock original design. wich may be why the power and distortion are differant

[@Rowuk2024]

To me it sounds like there was a preference for "distortion" in that group.

[@jungtarcph]

So refreshing with a guy below 40 🚀🚀💪💪👶👶 and I am not even gay, but lets get more young people!!

[@OscarSanchez-tk3hx]

Sadly when I went to a divorce process I left two Dynaco 70's.my ex kept the house and my car. Hey buddy thanks for the review

[@peterhakanson6797]

The vta needs changing feedback. Change 7.5k (r7) to 2k and distortion will be reduced and the "one note bass "on som speakers will disappear.

[@NebukedNezzer]

shows that lots of feedback gives better tests but not as good to listen to it. example my leak stereo 20 sounds better with lower gain(resistor changes)(all 12au7) and less feedback.

[@frankgeeraerts6243]

NO VTA for me ..........just one mod on the original ( independant bias for each tube )

[@melockavich9596]

you cants pick a song a a phone come on man. get a very good cd player and a very good preamp

[@jdmccorful]

Like the sound assessment. Also enjoyed instrument testing. Thanks.