EQUIPMENT FORUMS >> SE34I.5 >> Missing low freq's

Message started by funch on 11/03/13 at 04:19:07

Title: Missing low freq's
Post by funch on 11/03/13 at 04:19:07

I recently picked up a V-cap'ed, config. 1 Rachael on Audiogon to use
with my Audeze LCD2's. It's driven with a CSP2+. While the mid's
and highs are to die for, I'm noticing that the very bottom low's are
MIA, at least compared to my Taboo II. The output tubes are PSVANE
6CA7's; inputs are Mullard CV2493's. I've tried the stock tubes, but
no luck.

Any thoughts? Is it just the nature of the beast?

Title: Re: Missing low freq's
Post by Lord Soth on 11/03/13 at 04:52:35


Have you tried using other tube rectifiers?

I have encountered your problem before but with other tube equipment.

I suggest trying out a Philips 5R4GY from Upscale Audio.
If you are using the stock rectifier, it is likely to improve all freqs and not just the bottom end.

Title: Re: Missing low freq's
Post by funch on 11/03/13 at 04:56:30

I'm using a United USAF 596 rectifier. I do have the 5Y3 from Upscale,
as well as a couple of 5U4G's. I give them a try. Thanks.

Title: Re: Missing low freq's
Post by funch on 11/03/13 at 05:18:48

No luck with any of the rectifiers. I'm wondering if could be a slight
impedence mismatch with the 'phones?

Title: Re: Missing low freq's
Post by Lord Soth on 11/03/13 at 06:32:11

Using different types of rectifiers should change the operating point of your tubes.

Did any particular rectifier type make any improvement in the low end, even slightly?

Title: Re: Missing low freq's
Post by alper_yilmaz on 11/03/13 at 08:43:41

Hey funch,

I have had the same issue with my SE34I.2+.  I have received some suggestions when I posted the issue here.  I ended up going for a subwoofer, yet I realize I am still not happy with the bass in contrast to the amazing mids and treble.  At first, I thought I solved the issue but after hearing the bass on few friends' (even) mediocre systems at best, I was planning to write to Steve about this.

I am not familiar with your speakers, but mine are 97dB single driver Hoyt-Bedfords.  I do not have the impedance curve, but my guess is they reach a peak at lower frequencies and therefore there is an unevenness in the frequency response.  I have this problem even with my TORII mkII.  

I personally am considering having the EX mod on the SE34 which will make it more suitable with 6-16 ohm speakers.  I am not sure if this will be the ultimate solution, but the other potential solution that comes to my mind is having a two-way speaker with the least complex crossover (which is less likely to soak the power of the amp).

Folks, please chip in with tour two cents and correct me f I am talking gibberish here!  :)



Title: Re: Missing low freq's
Post by DBC on 11/03/13 at 14:14:53


I have the Super Zen and had similar issue. Mid's & High's to die for but light on the low end. With low watts just not enough power for the low end.

Added an HSU sub but did not fully solve the problem. Went with an HSU Mid Bass Module and problem solved. Read the thread "Adding Weight & Body" and I explain my theory as to why the Mid Bass Module works so well.

Title: Re: Missing low freq's
Post by funch on 11/03/13 at 22:40:32

Thanks for the replies guys. I don't use speakers. The Audeze LCD2
is a planar headphone, and the bottom end is definitely present with
the Taboo II, so it sounds like the issue might be an impedence
mismatch. The 'phones impedence is around 50 ohms.

What is the EX mod? I'm not familiar with it.

I know. I'll get out the soldering iron, and transplant Rachael's mid's
and highs into the Taboo.

Title: Re: Missing low freq's
Post by alper_yilmaz on 11/04/13 at 00:52:09

To the best of my knowledge, the EX mod simply changes the output transformers' impedance from 9600 ohms down to 3300 ohms (I might wrong about the exact figures) so that the amp can run speakers with higher impendance.  This causes lower output but with efficient speakers it still should be more than enough volume.

The mod is based on the fact that single-driver speakers with no crossovers tend to have fluctuating impedances across its frequency range.  It is highly likely that such a speaker has a peak in lower frequencies and therefore the lack of bass.

DBC's suggestion makes sense to me but I am looking for a solution without any subs or mid-bass enhancing designs.  



Title: Re: Missing low freq's
Post by funch on 11/04/13 at 01:24:01

Thanks. It sounds like I need to investigate the mod further.

I was also getting a fair amount of hum (not surprising through
headphones). I discovered the problem was of my own doing.
I use DIY IC's using Cardas cabling. The shield is connected on
one end, which should then be connected to the source end.
I discovered that I had both IC's connected 'backwards'. Flipping them
cleared up the hum by about 80%. It's tough being dumb.

Title: Re: Missing low freq's
Post by hifitubes on 11/24/13 at 10:30:34

I don't find the low-end lacking with Rachel, but I have a powered Deep Hemp sub that I can dial in. I'm thinking about running Mini Torii to sub when it get here. Glad you solved your hum problem!

Title: Re: Missing low freq's
Post by alper_yilmaz on 11/10/14 at 09:46:05

It has been more than a year since this thread had been started, but now that I am also using my SE34I.2+ with the Audeze LCD-2, I am observing a similar issue.  I do not think the issue is an impedance mismatch though.  I went through the LCD-2's impedance curve and it is pretty flat all the down to 32Hz and even below that (around 20Hz) it is only -4dB.  So practically, whatever the amp is pushing into the headphone should be represented equally (please correct me if my reasoning is not right).

In another thread, one was mentioning that KT88 has a fuller bass on Rachael, but I did not observe that with my JJ KT88s.  Maybe I should try another pair from a different brand.  I tried a pair of KT150s which has amazing bass response, but the amp cannot auto-bias with those tubes, so I ended up with a very dull sound.  I am not sure if I can get around that by getting rid of the auto-bias, as I am used to doing the biasing myself on my current rig.

Any other suggestions?  Best,


Title: Re: Missing low freq's
Post by Steve Deckert on 11/12/14 at 20:00:57

The SE34I.2 and SE34I.3 are single ended triode amplifiers designed for loudspeakers.  They will pair well with anything between 4 and 16 ohm.  This amp does not have optional output transformers like it's 2 watt little brother, the SE84CKC.

The amps are occasionally built with a headphone jack as a convenience for people who want to do both loudspeakers and headphones.

Since the LCD2 is 50 ohms, and you have no speakers hooked up, it is not a great surprise to see it have a rolled off bass response.  

One thing that is recommended for people who are not using speakers at the same time as headphones, is to "tune" your headphones by hooking a 5 watt resistor to each speaker jack on the amp.  You can try different values between 10 and 100 ohms.  


Title: Re: Missing low freq's
Post by alper_yilmaz on 11/13/14 at 10:18:32


This is very helpful; thanks!  That was my weekend project, but I was not sure about the value(s) for the resistors.  My LCD-2 is 70 ohms actually.  At least, that is what the web page says.  Should that change the value of the resistors?

A very basic question, but what does the resistor do on the signal path; does it simply make the speaker outputs more coherent with the headphone impedance?  In other words, rather than working with a suggested load of 4-16 ohms, does it simply make it more suitable for a higher impedance load?

Thanks for the information, again.  Best,


Title: Re: Missing low freq's
Post by alper_yilmaz on 11/13/14 at 22:25:30

OK, here is an update!

Being the impatient man of the block, I could not keep myself till the weekend and ended up in an electronics parts store late in the afternoon and bought a bunch of different 5W resistors.

First, I soldered the 47-ohm resistors.  Significant improvement in bass!  Aydin Esen's "Anadolu" started sounding much stronger in the bass compartment.  Anthony Jackson's bass on the opening tune and Dave Holland's on "Love's Haunts" were tighter and much more prominent than before.  Thom Yorke's most recent album full of synth basses was quite satisfying.  Herbie Hancock's version of "Norwegan Wood" also sounded fine in terms of the bass.  However, when I listened to Pat Metheny's "Bright Size Life", Beatles "In My Life" and Sting's "A Thousand Years" and "If I Ever Lose My Faith in You", I felt like the bass did not sound as it should have.  Particularly on Massive Attack's "Teardrop" the sub-bass performance was quite poor...

So, I unsoldered the 47-ohm resistors and soldered the 91-ohm ones.  These came with significantly better results than the previous ones.  I went over pretty much all the songs listed above and the results are quite fine.  Also I swapped the output tubes (Siemens EL34s) with Electro-Harmonix 6CA7s and they have also improved the bass slightly.

For the time being, I am happy with the outcome of the exercise.  Thanks for the very helpful suggestion, Steve.



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