Electro-Voice T35A
A 1.87-pound diecast horn tweeter that crosses at 3,500 Hz and never asks for more than five watts to do its job.
Overview
The T35A is small. It weighs 0.85 kg - 1.87 pounds - and measures 13.3 cm tall, 5.1 cm wide, and 6.0 cm deep. It is a horn, a magnet, and a diaphragm, and not much else. The diecast aluminum horn is finished in baked black enamel, and the whole assembly is small enough to sit in one hand. It is not a full-range driver and never pretends to be. It is a tweeter designed for use above 3,500 Hz in high-efficiency speaker systems, and everything about the design follows from that.
Frequency response runs 3,500 to 16,000 Hz within ±5 dB, measured axially at one meter with a swept sine wave. The usable low-frequency limit is 3,500 Hz, and the manual is blunt about why: a minimum crossover frequency of 3,500 Hz should be used to prevent entrance of lower frequencies harmful to the voice coil and diaphragm assembly. Feed it anything below that and you are asking for trouble. Sensitivity is the headline figure - 105 dB SPL at one meter with one watt of pink noise band-limited from 3,000 to 15,000 Hz. That is extraordinarily high. It will keep up with a high-efficiency woofer without an attenuator fighting it the whole way.
The power rating is 5 watts long-term, measured with a 4,000-20,000 Hz pink noise signal at a 6-dB crest factor for eight hours. Five watts sounds like nothing until you remember the 105 dB sensitivity. Five watts into this horn produces output that would take a hundred watts to match from a conventional dome. The manual also notes a 50-watt short-term rating, which is the number that matters for musical peaks rather than continuous thermal load.
Impedance is 8 ohms nominal, with a minimum of 7 ohms above 3,500 Hz - a gentle load that almost any amplifier will drive without complaint. The voice coil is 1.0 inch in diameter, wound around a phenolic diaphragm. The magnet is strontium ferrite, 0.23 kg, with a flux density of 1.35 Tesla. That is respectable for a magnet this small, and it is part of why the sensitivity is so high. The manual's architects' and engineers' specification describes the construction as a rugged phenolic diaphragm and a diecast aluminum horn. That reads like language written for a contractor specifying a sound reinforcement system.
The integral diffraction horn handles dispersion. Horizontal beamwidth is 95 degrees at 8 kHz; vertical beamwidth is 65 degrees at the same frequency. The directivity factor, Q, is 9.9 at 8 kHz. The manual notes that horizontal coverage is constant over the range from 4 kHz to 10 kHz, so the coverage pattern does not collapse into a narrow beam as frequency rises. The polar responses were measured in University's large anechoic chamber, with the microphone placed 6.1 meters (20 feet) from the horn mouth and rotation taken about the waveguide's geometric apexes. The manual notes that those axes are close to the apparent acoustic apexes across the measurement range, and that the long measuring distance reduces the error from the slight difference to an inconsequential level.
Mounting is not as simple as cutting a hole. The magnet structure is too large to pass through a baffle opening cut for the horn, so the T35A must be rear-mounted to the rear of the speaker baffle. The baffle opening needs to be 2.71 cm by 10.72 cm (1.68 inches by 4.22 inches) for best results, and the rear surface of the baffle should be routed out to reduce its thickness from the usual 1.9 cm (0.75 inch) to 0.63 cm (0.25 inch). The routed portion should be concentric with the tweeter opening and measure 5.72 cm by 14.0 cm (2.25 inches by 5.5 inches), which gives plenty of clearance for the horn. Skip the routing and the horn will not seat properly. This tweeter assumes you are building a cabinet, not dropping it into a pre-cut hole in a bookshelf speaker.
Wiring is straightforward - stranded wire of 22 gauge or larger connects the T35A within a two-way or three-way system. The manual recommends the University Sound X36 crossover for matching to full-range cone speakers and midrange components, and an AT38 level control to match sensitivity between components. The level control advice is specific: counterclockwise adjustment will normally be required to bring the tweeter down to match the woofer, and proper adjustment renders a more natural quality of sound. In plain terms, the T35A is so sensitive that it will dominate most systems unless you turn it down. It is easier to attenuate a hot tweeter than to boost a weak one, but it does mean the level control is not optional in practice.
The T35A appears in the surviving record under both the Electro-Voice and University Sound names, and the manual chunks here are attributed to University Sound. The relationship between the two brands is not documented in the available material, and it would be speculation to characterize it. What is clear is that the T35A is a horn tweeter built to a specific brief: high sensitivity, controlled dispersion, modest power handling, and a crossover point high enough to keep the diaphragm safe. It is a component for people who are building something, not buying something. The 105 dB sensitivity figure and the 3,500 Hz crossover point are the two numbers that define it, and everything else — the diecast horn, the phenolic diaphragm, the 1.35 Tesla magnet — exists to serve those two figures.
Period advertising for the T35 family leaned hard on the dispersion story. A run of advertisements in the mid-1950s led with the claim that the tweeter's coverage was impossible to obtain with another type of speaker, citing 3,500 cycles to beyond audibility, 180-degree dispersion, and a ±56 dB RETMA rating. Those ads ran in High Fidelity (February 1955, p.36), High Fidelity (May 1955, p.36), and Audio (October 1955, p.83). They are advertising copy rather than independent measurement, and the 180-degree dispersion claim in particular is a marketing figure, not the 95-degree horizontal beamwidth the manual specifies at 8 kHz. The ads do confirm the T35 family was being pitched to the same high-efficiency, build-it-yourself audience the manual addresses. The T35B is named alongside the T35 in those advertisements as the version for lower-powered systems and existing quality AM-FM radio or radio-phonograph setups, finished in flat matte black, requiring only an X36 to attach to a music system, with an AT37 level control optional. Whether the T35A and T35B differ only in finish or in something more substantive is not documented in the available material.
The T35A is not a wideband tweeter. The response stops at 16,000 Hz, and the ±5 dB tolerance means it is not ruler-flat within that band. The usable low-frequency limit of 3,500 Hz means it cannot be crossed over lower to meet a woofer with a natural rolloff in the 1-2 kHz range. It is a specialist component with a narrow job: turning a tiny amount of amplifier power into a lot of high-frequency output with a coverage pattern that stays consistent across its operating range.
eBay Listings
As an eBay Partner, we earn from qualifying purchases. This helps support our independent vintage technology research.