Monday, May 16, 2016

Altec 605B experiments



Hot on the trail from the 1" compression driver survey, I set forth to re-evaluate my open baffle mounted Altec 605B. This Duplex system is essentially an Altec 416A woofer + Altec 806A compression driver with an integral short horn. I wanted to address a mild honk and try to squeeze out more "air" in the top end from the horn/driver combo.

605B as a 416A woofer


Since my modus operandi has always been to listen carefully to each component as a separate entity, I disconnected the compression driver from the N1600C crossover and listened to the woofer rolling off at about 1100 hz as well as bypassing the crossover, in full range mode. 


After several days of listening with and without the crossover, I realized that the "honk" actually emanated from the woofer. Perhaps this was due to the typical midrange impedance rise in a woofer exacerbated in an open baffle by the lack of a true enclosure? So I winged and auditioned a few Zobel EQ values and ended up with 10 ohms + 11uf in parallel with the woofer terminals as the "best compromise." This circuit cost a couple of dB in efficiency but did not significantly affect dynamics while minimizing the honk. 

Towel damping

Not scientific, but hanging a towel behind the driver focused the sound a bit further ;)


+ Altec 32B horn



For kicks, I did another round of listening with the 605B acting as a woofer coupled to the Altec 32B horn using the same drivers (except the Beyma) and crossover options discussed in the previous survey. But unlike the 12" Altec 414A which has a smooth roll off characteristic, the 605B woofer had to be configured as outlined above. In this configuration, the woofer blended nicely with all four drivers. In fact the 605B woofer section + 32B/802D horn combo had a similar midrange tonal balance to the Altec 2-way. Could that be a 32B horn coloration? ;)

Altec 16 ohm diaphragms


The Altec 605B was factory fitted with the aluminum voice coil 20275 diaphragm. By the time I acquired it, the diaphragm has been replaced with the newer 34852 with a copper voice coil. I've swapped the 34852 and the 20275 (from my 802D) in and out of the 605B and always preferred the midrange oriented 20275 over the more extended high frequencies from the 34852. Thus, to my ears the 34852 and 20275 cannot be used as a stereo pair. Alas, NOS 20275s are very scarce but fortunately GPA is reproducing original 34852 diaphragms. Just to make sure my 34852 is an original Altec item I posted a query at the Altec Users' Board.

34852 vs. 20275

Having confirmed that my 34852 was genuine, I compared the JEL/N1600C XO to the stock N1600C in search of more sparkle and air from the 806A. The JEL/N1600C with EQ circuit did augment the top end a bit, but not to the same degree achieved by the other drivers with more powerful magnets used in the 32B horn experiment above. With or without the Zobel network, I found too much depression in the midrange of the 605B. This was most notable in the vocal range but not as audible with instrumental music. Switching to the 20275 diaphragm improved the midrange a smidgen but I was still not fully convinced that using the JEL/N1600C XO was the proper course.

Coda


Altec N1600C + Zobel EQ

For now I'll continue using the Altec 605B with the N1600C crossover with Zobel EQ + a sparkler...

Sparkler = Altec 3000H

Sunday, April 10, 2016

Stereo > Mono Line Level Mixer

Mixing line level stereophonic signals into full dimensional monophonic ;)


Here's a DIY project for mono aficionados that's so basic, I should have uploaded it many years ago. But other things got in the way.


Just like an MC step-up, these are entirely passive devices. 


The simplest way to convert stereo to mono is to connect left and right channels via a Y connector. But there's a more elegant way than just shorting two channels.


Resistors can blend two signals into one better by...


...using Allen-Bradley carbon composition for warmer sound ;)

We can end the mixing quest here but...


...the most elegant way of mixing two channels as was done in the studios by audio pros of yesteryears was through transformers.


This single UTC A-20 transformer was in a box lot of tubes and parts I picked up at a radio show. It's a high quality transformer designed for mic, mixer or line matching applications. Instead of trying to find a mate for MC step-up duty, I repurposed it for a greater calling in my mono rig.


With a claimed bandwidth of 10-50,000 hz and dual windings in the primary and secondary, it is perfect for mixing line level stereo signals into mono!

Left and right channels from a stereo source are fed into each primary winding. The summed mono output is taken from the series-connected secondary winding, which goes into the line level input of a mono preamp.


I also had a pair of 1:1 Tamura line level matching transformers. With the secondaries connected in series, it functioned like the UTC A-20. I tried an Altec 15095A but the step up ratio provided too much gain in my set up. It will work better in systems with lower gain.

Aside from mixing duties, transformers also serve as great digital signal processors. Who needs a DAC? ;)

Thursday, March 10, 2016

1" compression drivers - Emilar, Renkus-Heinz and Beyma

Is there life after Altec?



My first encounter with Emilar and Renkus-Heinz was in a discussion at the Lansing Heritage forum in 2006. Apparently, Jonas Renkus was a principal at both companies in the 70s and 80s. Prior to co-founding these companies, he worked under John Hilliard at Altec Lansing and followed him at Ling Temco Vought (LTV) where they developed a new compression driver with a polyimide suspension aluminum diaphragm. A paper on this research was submitted to the Journal of the Audio Engineering Society in 1966. This association piqued my interest, which triggered my search for affordable Emilar and Renkus-Heinz drivers.


Soon after I arrived in Manila, I became friends with artisanal speaker craftsman Lin Gomez. He is the go to person for Altec, JBL, Jensen, Western Electric or any vintage driver that needs restoration. His company used to be the official Beyma importer. During his closeout sale of Beyma inventory, among those I purchased was a pair of Beyma CP380M.

For the past couple of years, I've been playing with these drivers in the attic mounted either on the Altec 811, RCF H3709 and Altec 32B. But all of my recent critical listening was done through the mono rig driving one side of the Altec 2-way using the following crossover set up.


Crossovers


JEL/N1600C crossover + ATT/HF EQ
for 16 ohm drivers

The Altec N1600C series crossover has been a significant constituent in the evolution of my Altec 2-way horn speaker system. C. G. McProud's 1947 article on page 101 of Audio Anthology, Volume 1, was my first encounter with series crossovers although Western Electric was already using the topology in the 1930s. As an aside, my knowledge of electronics is basic. I don't even own a circuit simulation program and rely on my collection of audio electronic textbooks, old magazines plus what my ears tell me as reliable internet sources for "techie stuff."

Based on Earl K's simulations and analysis at the Lansing Heritage forum, Altec configured the N1600C 2-way crossover with staggered filters. In this application, a 2nd order/12dB/octave high pass filter @ ~ 1600 hz is provided by the 4 uf capacitor + 1mH inductor, while the 1mH inductor series connected to a 16 uf capacitor functions as a 1st order/6dB/octave low pass filter @ ~ 1100 hz. The 30 ohm potentiometer is part of an upper midrange frequency attenuator/high frequency EQ boost circuit I derived from reading through Altec 30923 discussions at Audio Karma and at Lansing Heritage. Altec incorporated this circuit in their Model 19 crossover and Jeff Markwart used it in his phase corrected Altec 605 crossover. 

In subjective terms, the combination of staggering the crossover point + upper midrange attenuator creates a "dip" in the crossover region where harshness and congestion can easily be detected by critical listeners, while my Altec 3000H tweeters were made redundant by the high frequency EQ boost.

During the critical listening test, I also rigged this crossover for the Altec 802D and Beyma CP380M (both 16 ohms) cutting in @ ~ 3200 hz with a 6dB/octave slope by using just the high pass section leaving the Altec 414A to run in full range mode.

3khz @ 6dB/octave high-pass + HF ATT/EQ
for 8 ohm drivers

Essentially this crossover is the same as the high pass section of the JEL/N1600C with R/C values for 8 ohm compression drivers. The 8 ohm L-pad substituted in place of the 30 ohm pot works just as well, and R/C values were adjusted accordingly for the attenuator/EQ.

It was used as a stand alone unit operating as a 1st order/6dB/octave @ ~ 3200 hz high pass filter with the Altec 414A naturally rolling off. Alternatively, when grafted in place of the JEL/N1600C high pass section, it worked as a 12dB/octave ~ 1600 hz high pass filter for an 8 ohm driver.


Splitting hairs


For all intents and purposes, these drivers were designed for similar real world applications. Since each manufacturer had their own set of measurement parameters, I deemed the inclusion of driver specifications inconsequential. Anyway, I noted that the minor differences in driver efficiency were well within the shelving range of the attenuator on both crossovers.

My main objective was simply to find out how each compression driver attached to the Altec 32B horn blends with the outstanding midrange performance of the Altec 414A woofer loaded inside the 614 bass reflex cabinet. In an ideal world, the blend should be seamless, with the horn/driver combo just filling in the missing upper frequency harmonic overtones not within the reach of the woofer.

Altec 802D
16 ohm original aluminum diaphragm
Alnico magnet

The 802D has been a musical companion for almost 20 years, so I know it very well. Attached to the 32B horn and playing duets with the 414A, they produce a harmonically rich and luscious midrange along with high frequencies that are silky smooth and extended. To my biased ears, it would be difficult to equal the 32B/802D/414A/614/JELN1600CXO quintet, short of a WE757A ;)

Emilar EC175-8
8 ohm original diaphragm
Ferrite magnet

My first impression of the EC175-8 which lingered all throughout my listening test, was a rich midrange reminiscent of the 802D. Perhaps this was due to its rather subtle presentation of high frequencies which are there, yet not stealing the limelight from the midrange. It's worth noting that this driver has a similar long throat design as the 802D. 

Renkus-Heinz SSD 1800-8
8 ohm original diaphragm
Ferrite magnet

The SSD 1800-8 + 414A was also a coherent combination. But the tonal balance was the antithesis of the EC175-8. The midrange sounded a bit leaner due to more prominent high frequencies. This driver has a short throat/pancake design like the 902-8B below. I wonder if the short throat configuration tends to enhance high frequencies?


Caveat: Based on my internet research, there are no original replacement diaphragms currently manufactured for Renkus-Heinz drivers.

Altec 902-8B
8 ohm original diaphragm
Ferrite magnet

To make things more interesting I borrowed a pair of Altec 902-8B cherished by my buddy Joel. The sound of this driver is similar to the 802D with the midrange not quite as lush but still vivid and involving. Since it had the best high frequency extension and transient response, the sound was invigorating. 

Top left: Beyma 8 AG/N 8" fullrange
Top right: Beyma SM115 15" woofer
Bottom left: Beyma CP380M
Bottom right: OB experiment ca. '09

In early 2009 I experimented with the CP380M + 32B coupled to the 15" Beyma SM115 woofer in an open baffle. I was actually quite impressed by the potential of this combination which was the reason I bought them.

Beyma CP380M
16 ohm polyester diaphragm
Ferrite magnet

However, in the context of this survey, the CP380M did not fare as well as the other drivers. Although the sound was quite pleasant with good high frequency extension, I was constantly aware of a transition between the 414A and the horn. Perhaps it was really voiced for a Beyma woofer?

Coda


The Emilar EC175-8 and Renkus-Heinz SSD 1800-8 have their virtues and omissions. If their virtues were combined, then we have a true Altec alternative. Yet despite their minor flaws, mounted on the Altec 32B horn, both easily coalesced with the Altec 414A woofer which corroborated their genetic heritage. If I didn't have the 802D, I could have easily lived with either the EC175-8 or SSD 1800-8 with a slight preference for the EC175-8 just because I'm a midrange hedonist;) 

With regards to the Altec 802D vs. Altec 902-8B, it boils down to a matter of taste as well as availability and affordability. As for the Beyma, when time permits, I will rehash the OB/SM115 + CP380M/horn experiment and post my findings;)

Happy listening!

Sunday, February 28, 2016

Austin Mini Maintenance Woes


As soon as I got the brakes sorted...


...I had the exhaust leak fixed. 


In preparation for the summer heat, I installed the 
heater matrix 
to function as an auxiliary radiator.


I was about to take out the clutch master cylinder 
to clean up the rust, but it was too late.

Now I have to wait a few weeks for a replacement clutch master cylinder ;(